System for determining network route quality using sequence numbers

ABSTRACT

A system and method for determining route quality in a network in which multiple routes exist between a transmitting end-point and a receiving end-point. Initially, a routing sequence number is associated with each transmission sent on a route in the network, such that consecutive transmissions sent on one of the routes have consecutive routing sequence numbers. The routing sequence number for a first one of the routes is included in transmissions sent on a second one of the routes. The routing sequence number of the first route, received on the second route, is used as an indicator of the route quality of the first route, and this indicator is compared to an expected routing sequence number for the first route to determine a likelihood of whether the transmissions sent via the first route are either lost or delayed. The expected routing sequence number is normally incremented each time a transmission is received on the second route.

RELATED APPLICATIONS

This application is related to the following commonly owned and co-filed U.S. patent applications Ser. No. filed Apr. 30, 2004 and incorporated herein by reference: System And Method For Flow Control In A Network, Attorney Docket Number 200209199-1; System For Addressing Network End-Points Using Route Handles, Attorney Docket Number 200209216-1; System For Selecting Routes For Retransmission In A Network, Attorney Docket Number 200209219-1; and System And Method For Message Routing In A Network, Attorney Docket Number 200209220-1.

BACKGROUND

Whenever a network has multiple routes available for transmitting messages, a specific route must be selected for a particular message transmission. A number of problems exist with respect to selection of the best route for transmitting a message. While it is common to address route selection by computing quality of service metrics, it is challenging to do so without adding transmissions that are used solely for testing route quality.

Most existing routing algorithms focus on minimizing the number of hops between the two end-points rather than measuring latency or reliability. In systems that have attempted to compare route latency, transmission of additional messages has typically been required, but this technique increases route congestion as well as route processing overhead.

Selection of the route with the lowest hop count is a commonly implemented method. However, in many networks, such as Ethernet and Fibre Channel, the hop count of a route is not as important as the latency or frame loss rate of a route. Selection of the fastest and most reliable route, rather than the route merely having the lowest hop count, is thus desirable.

Communication within a prior art network 100, such as a storage area network (SAN), is illustrated in FIG. 1, which shows applications 101(*), running on end-points 102(*), communicating with their peer applications 101(*) via respective ports 103(*) and fabrics 105(*). As used herein, a ‘wild card’ indicator “(*)” following a reference number indicates an arbitrary one of a plurality of similar entities. The communication infrastructure in a storage area network often includes two or more independent Fibre Channel fabrics 105(1) and 105(k) comprising multiple switches and hubs (not shown). Each end-point 102(*) is connected to each Fibre Channel fabric 105(*) via one or more nodes 104(*), and may use any available fabric to communicate with other end-points 102(*). Each node 104(*) provides at least one port 103(*) between an end-point 102(*) and a Fibre Channel fabric 105(*).

A node 104(*) is a module such as a Fibre Channel adapter module, or a collection of circuits embedded within the end-point. As shown in FIG. 1, a node 104(*) may use multiple ports 103(*) to connect to a fabric 105(*); for example, node 104(1) includes ports 103(1) and 103(3), both of which are connected to fabric 105(1).

SUMMARY

A system and method are disclosed for determining route quality in a network in which multiple routes exist between a transmitting end-point and a receiving end-point. Initially, a routing sequence number is associated with each transmission sent on a route in the network, such that consecutive transmissions sent on one of the routes have consecutive routing sequence numbers. The routing sequence number for a first one of the routes is included in transmissions sent on a second one of the routes. The routing sequence number of the first route, received on the second route, is used as an indicator of the route quality of the first route, and this indicator is compared to an expected routing sequence number for the first route to determine a likelihood of whether the transmissions sent via the first route are either lost or delayed. The expected routing sequence number is normally incremented each time a transmission is received on the second route.

In a further embodiment, the expected routing sequence number is computed by increasing the expected routing sequence number by 1, if a received transmission describes the routing sequence number of the receive route and described routing sequence equals the current expected routing sequence number of the described route; increasing the expected routing sequence number by a value of N+1, if the received transmission describes the routing sequence number of the receive route and the described routing sequence number exceeds the current expected routing sequence number of the described route by a value of N; and increasing the receive route expected routing sequence number by 1, if the received transmission does not describe the routing sequence number of the receive route.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing applications, running on end-points, communicating with peer applications via respective ports and Fibre Channel fabrics over a prior art network;

FIG. 2 is an exemplary diagram showing the relationship between TCP/IP protocol layering and protocol layering of the present system;

FIG. 3 is a diagram showing relationships between exemplary physical components of a route in the present system;

FIG. 4 is a diagram showing relationships between exemplary logical objects in the present system;

FIG. 5 is a diagram of a route showing half-routes;

FIG. 6A is a diagram showing an exemplary relationship between routes and route sets;

FIG. 6B is a diagram showing exemplary relationships between a route set management connection, a route set, and application connections;

FIG. 6C is an exemplary diagram showing the relationship between a route management connection and pseudo end-points;

FIG. 7 is a diagram showing an overview of an exemplary set of steps performed in operation of the present system;

FIG. 8 is a diagram showing exemplary steps performed in establishing a path and a route management connection between two ports;

FIG. 9 is a diagram showing exemplary steps performed in a full route registration procedure;

FIGS. 10A, 10B, and 10C are flowcharts showing exemplary steps performed in establishing a route set;

FIG. 11 is a flowchart illustrating exemplary steps performed in handling a request to establish a connection between end-point applications;

FIG. 12A is flowchart illustrating exemplary steps performed in selecting a route for initial transmission;

FIG. 12B is flowchart illustrating exemplary steps performed in selecting a route for a retry transmission;

FIG. 13 is a diagram showing exemplary steps and queues used in queueing route selection requests;

FIG. 14 is a diagram showing an exemplary routing header;

FIG. 15A is a diagram illustrating exemplary routing layer processing performed in sending initial transmissions;

FIG. 15B is a diagram illustrating exemplary routing layer processing performed in sending retry transmissions;

FIG. 16 is a diagram illustrating exemplary routing layer processing performed in receiving transmissions;

FIG. 17 is a diagram showing an exemplary routing layer feedback loop used in establishing the transmission rate limit for a particular half route;

FIG. 18 is a flowchart showing an exemplary algorithm for determining whether a missing transmission is more likely to be lost or delayed;

FIG. 19 is a flowchart illustrating an exemplary method of determining expected routing sequence numbers;

FIG. 20 is a flowchart illustrating an exemplary method for computing transmission loss likelihood and transmission rate limit;

FIG. 21 is a graphical illustration showing an exemplary method for determining transmission delay;

FIG. 22 is a flowchart showing an exemplary method for calculating the mean delayed transmission expected receive time;

FIG. 23 is a flowchart showing an exemplary method for calculating transmission delay likelihood and how it is used to modify the transmission rate limit;

FIG. 24 is a graphical illustration showing an example of a delay skew sum calculation; and

FIG. 25 is a flowchart showing an exemplary algorithm that uses the algorithms of FIGS. 19, 20, 22, and 23 to determine transmission rate limit.

DETAILED DESCRIPTION

Definitions

The following definitions are applicable to the present document:

End-point—a locus for execution of applications on a network.

End-point Incarnation—the sustained, continuous operation of an end-point without loss of context.

Connection—a relationship between two communicating program incarnations, or processes, that is maintained where those processes are running. While the connection is established, the processes may use it to communicate. If either of the communicating processes stops (i.e., fails, exits, etc.) then the connection fails, and if restarted, the programs cannot continue to use the previously existing connection. Within this document, the following examples of connections are described:

-   -   An application connection is a relationship between two         incarnations of an application that is maintained by the         end-points where those applications are running and that allows         them to communicate reliably.     -   A route set management connection is a relationship between         routing layer programs on two end-point incarnations that allows         them to communicate route set management information reliably.     -   A route management connection is a relationship between two port         layer programs on two port incarnations that allows the reliable         communication of route management information.

Port—a port comprises all network-specific functionality associated with a specific, single Nx_port (Fibre Channel N_port or NL_port). Also denotes a specific communications protocol layer.

Message—A logically contiguous array of bytes sent reliably by the sequenced message transport. A message is sent using one or more transmissions.

Transmission—A single-frame Fibre Channel sequence having, at least, a Fibre Channel header, network header, and a routing header. Transmissions are sent on a best-effort basis.

Outbound Message—an object that is used to describe a message to be sent. Acronym is OBM.

Outbound Sequence—an object that is used to describe a transmission to be sent. Acronym is OBS.

Path—a relationship between two port incarnations established by the standard Fibre Channel PLOGI extended link service.

Process—an incarnation of the code on a machine that is executing the code.

Route—a connection between the processes that represent the two end point incarnations that uses a specific path. A route relates two end-point incarnations and two port incarnations.

Half route—the portion of a route that delivers messages in one direction. With respect to an end-point, a half route is either outbound or inbound. A route consists of exactly two half routes.

Route set—a relationship between two end-point incarnations that associates the complete set of routes between those incarnations and that indicates the existence (currently, or at some previous time) of one or more routes between the end-points. A route is a member of exactly one route set. A route is established as a member of a specific route set and cannot migrate to any other route set. Termination of a route set terminates all routes belonging to the route set.

Introduction

FIG. 2 is a diagram showing, as an example, the general correspondence between TCP/IP protocol layering 20 and the protocol layering 200 of the present system. As shown in FIG. 2, each end-point 102 in the present system includes network layer 240, routing layer 230, transport layer 220, request layer 212, and application layer 210 plus a buffer pool management component 270. An API (Application Program Interface) 211 makes network services available to applications 210. Network layer 240 encapsulates one or more ports 103(*) that are local to the network layer's end-point 102 and makes them available to that end-point. In so doing, the network layer 240 hides any physical interface (e.g., a PCI bus) between the end-point 102 and its local ports 103(*).

The present system uses a routing layer 230 to effectively glue together the network 240 and transport 220 layers of a networking protocol. Routing layer 230 may be implemented in conjunction with Hewlett-Packard's SCTP (Storage Cluster Transport Protocol), for example. The routing layer 230 provides a routing or steering mechanism to direct outgoing transmissions from a connection onto the proper route, and to direct incoming transmissions (on a particular route) to the proper connection. Transport layer 220 sends and receives messages on connections and provides a sequenced-message delivery service. Request layer 212 provides the command-response service, command flow-control, and the bulk data transfer service. A buffer pool management component 270 manages buffers (not shown) used to receive unsolicited messages directly into application memory. An API (applications programming interface) 211 makes network services available to applications 101(*).

The functionality that is specific to a single port 103 is called the port layer 250. In the present exemplary embodiment, port layer 250 performs the discovery and maintenance of routes, in addition to providing mechanisms for sending and receiving transmissions as described herein. In the present embodiment, each port 103 comprises an ‘Nx_port’, which is a Fibre Channel N_port or NL_port. Each port 103 uses a driver (‘Fibre Channel services’) 260 to abstract the port hardware such that the peculiarities of the Fibre Channel port hardware 261 are hidden from the upper protocol layers. In an exemplary embodiment, an end-point 102 may use several ports 103(*) simultaneously.

As shown in FIG. 2, the application layer 21 in the TCP/IP protocol layering scheme 20 may be considered to correspond to the application and request layers 210/212 of the present system. In the present protocol layering scheme 200, the TCP/IP transport layer 22 is functionally similar to the present transport layer 220; the TCP/IP network layer 24 may be considered to correspond to the combination of the present network layer 240 and port layer 250; and the TCP/IP link layer 25 is effectively implemented within Fibre Channel Services and port hardware layers 260/261. While the TCP/IP protocol does routing within its network layer 24, it does not specifically provide for a distinct formal routing layer.

In the present exemplary embodiment, the protocol stack 200 provides a reliable one-way sequenced message delivery service for small messages, a reliable command-response service that uses the sequenced message delivery service to deliver commands and responses, and a reliable, high-performance bulk data transfer service that can be used in conjunction with the reliable command-response service.

The transport protocol used to implement transport layer 220 allows two application processes running on end-points 102(*) to establish a connection between them and to use that connection to send one-way sequenced messages. Barring major communication failures, messages sent via the connection are delivered in order and exactly once to the connected process. A command-response service implemented by the request layer 212 (using services provided by the transport layer 220) allows a client process, e.g., application 101(1), to send commands to a server process, e.g., application 101(2), for the server process to return a response to the client process, and for the client and server to perform high-performance bulk data transfers. Commands and responses are sent via a connection established between the client and the server processes, so they are presumed by the request layer to be delivered reliably.

As can be seen from FIG. 2, the present system adds a routing layer 230 to an end-point's protocol stack between the transport 220 and network 240 layers. This routing layer 230 is aware of multiple routes and networks to other end-points 102. As described herein, the routing layer 230 organizes the available routes to a given end-point 102(*), measures route quality, and selects the proper route for each outgoing transmission.

The present system's transport layer 230 uses different routing layer functions for sending initial transmissions and retry transmissions. This distinction allows the route selection for retry transmissions to differ from route selection for initial transmissions. In addition, transport layer 230 informs the routing layer when a previous transmission may not have arrived in a timely fashion so that the routing layer can avoid the route used by the previous transmission.

As an adjunct to the routing process, routing layer 230 monitors the quality of each route. The resulting route quality is used to select routes for outgoing transmissions to avoid routes that are unreliable, congested, or slow.

Routing layer 230 automatically maintains the routes in a route set (described in detail below), adding newly discovered routes and removing those that have failed.

Route Components

Physical components governed by the present system include end-points 102(*), ports 103(*), and fabrics 105. Certain relationships between these component objects are summarized in FIG. 3. As shown in FIG. 3, a route 300 comprises a pair of ports, e.g., 103(L, 1) and 103(R,j), connected via a fabric 105 through which two end-points, e.g., 102(1) and 102(2), can communicate. In the above figure, there are i*j potential routes 300 between end-point 102(1) and end-point 102(2), j through each port [103(L, 1) through 103(L, i)] directly connected to end-point 102(1). One of these routes is shown by the bold line 300.

The protocols described herein govern the creation, modification, and deletion of logical objects (structures). These logical objects describe discovered physical ports 103 and end-points 102, and include local port, remote port, local end-point, and remote end-point objects, which are described in detail with reference to FIG. 4. Route, route set, discovered remote end-point, and route management connection objects are used to group and manage instances of these logical objects, as described below. The term ‘local’ is used herein to refer to logical objects that represent physical objects that can be accessed without using Fibre Channel messages. In contrast, the term ‘remote’ is used to refer to objects that are not local. Thus, a remote end-point describes an end-point that can only be accessed across a Fibre Channel, e.g., end-point 102(2) is remote to port 103(L, 1), whereas end-point 102(1) is a local end-point relative thereto.

FIG. 4 is a diagram showing relationships between exemplary logical component objects (structures) and their location in port memory 420 and end-point memory 421, located in a physical port 103(*) and a physical end-point 102(*), respectively. In the present system, several objects (local port, route, local end-point, and requested remote end-point) are represented by the combination of a master and a proxy. This bifurcation into masters and corresponding proxies allows these objects to be distributed between the port memory/processor and the end-point memory/processor. By maintaining both the masters and proxies, the present system will support either a distributed implementation or a single memory/processor implementation. FIG. 4 is presented at this point to clarify subsequent references made to these system objects throughout this document. A brief description of certain objects shown in FIG. 4 is presented below.

Local port objects, comprising local port master 404 and local port proxy 413, describe the state of a directly accessible physical port. Local port objects 404/413 are created automatically at initialization based on physically detected port hardware. Attributes of a local port object describe the physical port hardware and its fabric login state.

Remote port object 401 describes the state of a local port's relationship with a physical port that is not directly accessible. Remote port objects 401 are created when they are discovered via Fibre Channel communication. Attributes of the remote port object 401 describe its port ID, port name, a local port through which it can be accessed, and port login state.

A local end-point object (e.g., local end-point incarnation master 414 and local end-point incarnation proxy 411) describes the state of a directly accessible end-point. Local end-point incarnation master object 414 is created when it is initialized on that end-point and the local end-point incarnation proxy is created when the local end-point discovers the port. Attributes of the local end-point object include the local end-point's UID (unique identifier) and IID (incarnation identifier).

Pseudo local end-point object 402 describes the local end-point for route management connections that is associated with a local port object 404. It is created as a side effect of creating the local port object 404. Likewise, a pseudo remote end-point object 403 is created as a side effect of creating a remote port object 401. Unlike real end-points, pseudo end-points are not addressed by their UIDs; rather, they are addressed by their associated ports during connection establishment and by route handles thereafter.

A remote end-point object describes the state of an end-point that is not directly accessible. Discovered remote end-point objects 407 are created when they are discovered through partial route registration, and requested remote end-point master objects 418 are created when a client makes a connect request to a new end-point 102. Requested remote end-point proxy objects 415 are created when either when a client makes a connect request or when a local port 103(L) is discovered.

Attributes of the remote end-point include the remote end-point's UID. Attributes of the discovered remote end-point object 407 include references to remote ports that can be used to access the remote end-point. Attributes of the requested remote end-point object (415/418) include references to local end-points that have requested that routes be established to the remote end-point.

An end-point incarnation 410/411 is the sustained, continuous operation of an end-point without loss of context. Attributes of an end-point incarnation include its incarnation identifier (IID). The incarnation identifier is assumed unique over all end-point incarnations. Each time an end-point reboots, its old incarnation is destroyed and a new one is created, including a new incarnation identifier.

Route management connection object 405 describes a connection that is used to communicate route management information between two pseudo end-points and their corresponding ports.

Routing layer connection object 416 is a base class from which a transport layer connection object is derived.

Route set object 417 associates the routes from a local end-point incarnation 411 to a remote end-point incarnation 410. Attributes of the route set include a list of routes, a list of clients (connections), and a remote end-point UID and incarnation identifier (IID) (the local end-point is implicit).

There are at least two viable route management configurations for the present system. In the first configuration, all local port and end-point objects are stored in a single memory and the processors that implement the local port and end-point methods have a method for synchronizing access to that memory. In the second configuration, the port and end-point objects may be distributed between memory and processors associated with ports 103(*) and end-points 102(*).

When the port and end-point objects are distributed, those objects are decomposed into a corresponding master object and its proxies. For example, local end-point incarnation 411 is decomposed into a local end-point incarnation master object 414 in the memory 421 of the end-point 102 and multiple local end-point incarnation proxy objects 412 in the memories 420 of the end-point's local ports 103(L). The local end-point incarnation master 414 is created automatically during the end-point initialization, and the local end-point incarnation proxy 412 is created as a result of end-point registration with a port 103. Similarly, the local port master object 404 is created automatically during port initialization, and the local port proxy object 413 is created during hardware discovery and modified as a result of end-point registration with the port 103.

Half Routes

Each route 300 comprises two independent, unidirectional, half routes. FIG. 5 is a diagram of a route 300 showing the two component half routes 500(1) and 500(2). From the perspective of an end-point 102(*), one of these components is the outbound half route and the other is the inbound half route. The local end-point's outbound half route is the remote end-point's inbound half route and vice versa. For example, if end-points 102(L) and 102(R) are considered to be the local end-point and remote end-points, respectively, half route 500(1) is the outbound half route for local endpoint 102(L), and half route 500(2) is the inbound half route for local endpoint 102(R).

Route Sets

FIG. 6A is a diagram showing the relationship between routes 300(*) and a route set 600. A route set 600 specifies the relationship between two end-points 102(*) and catalogs the routes 300(*) between the two end-points. A route set typically contains multiple distinct routes. As shown in FIG. 6A, route set 600 is the set of routes 300(1), 300(2), 300(3), and 300(4) between the two end-points 102(L) and 102(R). A route set 600 thus groups all routes 300 between a local end-point and a remote end-point. Routing layer 230 is responsible for creating, deleting, and maintaining route sets 600.

Before application processes on two end-points 102(*) can create a connection, the end-points must first establish a route set between them. A route set 600 may persist while routes are added to or removed from the route set, provided the end-point incarnations 410/411 continue to exist.

Route Set Management Connection

FIG. 6B is a diagram showing exemplary relationships between a route set management connection (RTSMC) 602, a route set 600, and application connections 601(1)-601(3), with application processes P1-P5 running on end-points 102(L) and 102(R) and communicating via application connections 601(1)-601(3) between the processes. A local end-point, e.g., 102(L), creates routes 300, a route set 600, and a route set management connection to a remote end-point, e.g., 102(R), when it finds it necessary to create a first application connection 601(1) with an application on the remote end-point.

Route Selection

In selecting a route 300 for an outgoing transmission, transport layer 220 implicitly constrains the selection to a route set 600 by specifying a connection on which to send a transmission. Routing layer 230 attempts to select a route 300 from that route set 600 that is reliable, not congested, and fast. When multiple routes 300 appear to be acceptable, then the routing layer 230 distributes traffic among those routes to balance their utilization.

Routing layer clients (i.e., transport layer entities) send and receive messages via connections established by the routing layer 230. These connections are base-class connections from which transport layer connections are derived. Routing layer 230 provides functions that clients can use to request and abort connections. A route set management connection 602, is used by the routing layer to manage the routing layer connections established on a route set. Route set management connections 602 are described below in detail with respect to FIGS. 10A, 10B and 10C. Once a routing layer connection is established, the routing layer connection's send and receive functions are available to routing layer clients.

Operational Overview

FIG. 7 is a diagram showing an exemplary set of high-level operations performed in operation of the present system. Much of the process shown in FIG. 7 is event-driven, and thus the execution of each functional block shown does not occur automatically in response to execution of a previous block in the figure.

Configuration Discovery

As show in FIG. 7, at step 702, port initialization begins with the fabric login and name server registration as described in the Fibre Channel specification. In order to provide better scaling for the discovery process, the subject protocols allow two classes of Fibre Channel ports 103(*), Class A and Class B. Class A ports register with the fabric name server as two FC4 types, while Class B ports register as only one of these two. In this way, Class A ports autonomously locate all of the Class A and Class B ports by querying the fabric name server for all ports that have registered the FC4 type used by all ports that support the subject protocols. In contrast Class B ports locate only Class A ports by querying only the FC4 type that Class B ports do not register. Class B ports can locate other Class B ports by querying a protocol-specific name server that is implemented by all Class A ports.

Once registration is complete, port configuration discovery is performed periodically, at step 705, with each local port 103(*) querying the Fabric name server (logging in if necessary) to discover newly connected ports that have registered the FC4 types used by the ports that support the subject protocols. In the steady state, configuration discovery is performed every 100 seconds, in an exemplary embodiment. However, during startup and whenever there is evidence that the system configuration may have changed, this period is changed, for example, to 10 seconds for 10 periods and then back to 100 seconds. It should be noted that the present system does not require setting the configuration discovery period precisely to the foregoing values.

This autonomous configuration discovery must be repeated periodically because fabric name server registrations propagate slowly, configurations change, and state change notifications are not delivered reliably. When repeated periodically, only newly discovered ports 103(*) are processed. This is true because ports that restart or whose IDs change will spontaneously execute portions of autonomous configuration discovery and thus update configuration changes.

Port Login

Port login is used to establish a shared context between two Fibre Channel ports 103(*). Prior to port login, the ports may only exchange port login transmissions (i.e., PLOGI/PLOGI_ACC transmissions). Successful port login establishes a path between the ports 103(*). Port login is specified by the Fibre Channel specification. Each port maintains a set of remote port objects (structures) 401, each of which describes the relationship between it and another port that it can access via a fabric 105(*).

As shown in FIG. 7, at step 710, a port 103(*) creates a path between itself and each remote port 103(*) that it discovered from the Fabric name server. A route management connection 603 (shown in FIG. 6C) is then established between the pseudo end-points 402/403 associated with the ports of a path, at step 715. Route management connection 603 is a sequenced-message connection that provides a one-way sequenced message delivery service used to communicate route management information between two ports 103(*).

FIG. 8 is a diagram showing exemplary steps for establishing a path and a route management connection 603 between two ports 103(*), and also for performing a ‘partial route registration’ process. As shown in FIG. 8 (with reference also to FIG. 3), at step 805, local port 103(L) requests that Fibre Channel Services establish a path to remote port 103(R) using a PLOGI transmission. Upon receipt thereof, a path to the remote port 103(R) is established, at step 810, and the remote port responds by sending a PLOGI_ACC transmission to the local port 103(L), at step 815. After receipt of the PLOGI_ACC transmission at the local port, a path from the remote port 103(R) to the local port 103(L) is established, at step 820.

At step 825, the local port then uses the Report Node FC-4 Types (RNFT) protocol to learn the set of FC-4 protocols supported by a particular remote port. Receipt of RNFT_ACC from the remote port 103(R) at step 830 indicates that the remote port supports the proper Fibre Channel FC-4 protocol, and causes the local port 103(L) to initiate process login.

Route Management Connection Establishment

The present system extends every local and remote port object 404/401 by attaching to it a pseudo end-point object 402/403 to provide a terminus for route management connections. FIG. 6C is an exemplary diagram showing the relationship between pseudo end-points 402/403 and a route management connection (RMC) 603. The present process login protocol establishes a route management connection 603 and a route 300 between pseudo end-points 402/403 associated with the ports 103(*) of a path.

The route management connection 603 is used to communicate:

-   -   partial route registration messages,     -   full route registration requests and their responses, and     -   name service queries and their responses.

Route management connection 603 behaves slightly differently than other connections. Differences include the following:

-   -   the two end-points of a route management connection 603 are         pseudo-end-points that correspond to ports of a path.     -   the route management connection 603 does not use a route set.

Rather, the corresponding path constitutes the sole route used by the route management connection.

-   -   when a remote Nx_port is implicitly or explicitly logged out,         all corresponding route management connections 603 are         terminated immediately.

The present process login protocol comprises an FC-4 Link Service request (SCPRLI) and an FC-4 Link Service reply (SCPRLI_ACC). The local and remote ports exchange process login protocol transmissions SCPRLI (step 835) and SCPRLI_ACC (step 840) to establish a route management connection 603 between the path's local and remote ports 103(L)/103(R), at step 845. SCPRLI and SCPRLI_ACC correspond to the first two phases of a three-way handshake. Information carried by the SCPRLI/SCPRLI_ACC FC-4 Link Service includes the route management connection's connection identifier (connection ID) and the route's full route handle, which is used to direct messages to a remote end-point 102(*) via a route 300, once established. The first message to be sent on the route management connection is a SCRPR command, at step 850, which completes the three-way handshake and completes establishment of the route management connection 603, at step 855, thus enabling the transmission of messages on the route management connection by the remote port 103(R).

More specifically, during process login, a local port 103(L) and a remote port 103(R) perform the steps below to establish a route management connection:

(1) On the local port, a route management connection object 405 and pseudo remote end-point object 403 are created that reference the remote port's remote port object 401.

(2) On the local port, a route master object 406 is created, and a SCPRLI command is sent to the remote port 103(R), as indicated above. The SCPRLI message contains the inbound full route handle, a connection ID, an initial sequence number, and flags describing the local port 103(L).

(3) On the remote port, receipt of a SCPRLI message normally modifies the states of the existing route management connection and route master objects 405/406 to the pseudo remote end-point 403. In the case where SCPRLI is received and those objects do not exist, they are created. Successful completion is signaled by sending a SCPRLI_ACC message containing the remote port's inbound full route handle, a connection ID, an initial sequence number, and flags describing the remote port 103(R).

(4) On the local port, receipt of a SCPRLI_ACC message modifies the states of the route management connection and route master objects 405/406 to the pseudo remote end-point 403 and establishes the route management connection 603.

Partial Route Registration

Register Partial Routes (SCRPR) messages are sent any time there is a change in the set of end-points that are local to a port, to allow end-points 102(*) discover one another. Once process login is complete, a SCRPR message is sent as the first sequenced message on the route management connection 603. The message includes the remote port's inbound full route handle and the connection ID. Each port 103(R)/103(L) uses the route management connection's one-way sequenced message delivery service to supply a list of its local end-points' UIDs and IIDs to the corresponding remote port 103(L)/103(R) using the partial route registration protocol (SCRPR), at steps 850/865. In response, the receiving port creates or modifies a discovered remote end-point object 407 for each listed end-point and registers it in its name server database. Subsequently, an end-point 102(*) local to the receiving port can discover the set of remote end-points 102(*) to which it can communicate via yet-to-be established routes 300 and connections 601. As a side effect of using the route management connection's sequenced message delivery service, each SCRPR message is acknowledged (at steps 860/870); i.e., the route management connection 603 assures that every sequenced message is ACK'd.

As shown at step 720 in FIG. 7, one or more routes 300 are established between end-points 102(*) using a full route registration protocol. At step 725, a route set 600 is established between the end-points 102(*). Steps 720 and 725 are described in detail in the immediately following section.

Full Route Registration

A port 103 uses full route registration protocols to establish and maintain routes 300 between its local end-point incarnations 412 and those remote end-point incarnations 409 that correspond to the logical intersection of discovered remote end-points 407 and requested remote end-points 415. These full route registration protocols include protocols to establish (register) and destroy (deregister) routes 300. Each active route 300 provides a mechanism for delivering transmissions between its two end-points 102(*).

Full route registration and deregistration cause the creation and deletion of route objects (route masters 406, route proxies 408, route sets 417, and route set management connections 419). As a side effect, remote end-point incarnation proxy objects 409 may be created and deleted. A remote end-point incarnation proxy object 409 is used to represent the remote end-point incarnation of a full route 300.

A remote end-point incarnation may be represented as an attribute of a route master object 406 (described below in detail), but because route master objects are relatively large, it is desirable to share them. Thus, in an exemplary embodiment, a remote end-point incarnation proxy 409 is created whenever a route master's remote end-point incarnation 410 is set to a previously unknown value, and deleted whenever no route master 406 references it.

The full route registration protocol comprises sending a Register Full Route message (SCRFR) from a local port 103(L) to a remote port 103(R) and a Register Full Route Response message (SCRFR_RSP) that the remote port returns to the local port. Both the SCRFR and the SCRFR_RSP messages are sent on the route management connection 603 associated with the route's path.

FIG. 9 is a diagram showing exemplary steps performed in a full route registration process. As shown in FIG. 9, full route registration is initiated after an end-point 102(L) has expressed a demand for routes to a remote end-point 102(R), at step 905, by issuing a Request Remote End-Point command to a local port 103(*). More specifically, full route registration is triggered either by an end-point 102 issuing a Request Remote End-Point command for an end-point 102 to which a partial route has already been discovered, or by the discovery of a partial route to a remote end-point 102(R) that was previously requested. In the process of establishing a route 300, two ports 103(*) exchange full route handles to use to address messages to the end-points via the route.

After full route registration is triggered, the local port 103(L) first checks to see if a route master 406 already exists to the remote end-point incarnation 410 that represents either an established route or a route that is in the process of being established, and if one exists then no further action is taken. Otherwise, a route master 406 is created to track the progress of full route registration and it is linked with the remote end-point incarnation proxy 409 and the route management connection 405. Then, the local port 103(L) changes the route master state to NRO and notifies the local end-point 102(L) that requested the remote end-point 102(R) by sending a New Route Originator (NRO) event, at step 910. The local end-point 102(L) either refuses the new route 300 or approves it and provides the local port 103(L) with the originator's ep_info (end-point information) structure, containing end-point information for the local end-point, to transmit to the remote end-point 102(R). A New Route Originator (NRO) event is generated by the route master object on the port originating the SCRFR to inform the local end-point that a new route to the requested remote end-point 102(R) specified by a remote end-point UID has been discovered through the path specified by the remote port 103(R).

When a new route is approved, the local port 103(L) looks up the route master, copies the ep_info and route proxy handle into the route master, and updates the route master state to SCRFR_SENT. The local port then allocates an OBM and uses it to construct a SCRFR message which it sends to the remote port, at step 915 via the route management connection. The SCRFR message conveys the route's local and remote end-point UIDs and IIDs, the originator's ep_info structure generated by the local end-point, and a full route handle that can be used to direct messages from the remote end-point 102(R) to the local end-point 102(L) via the route 300. In the case where the new route is refused, the local port runs down the route master, and no further action is taken.

On receipt of the SCRFR message, the remote port 103(R) checks to make sure that the remote end-point 102(R) referenced in the SCRFR message has registered with the port, and if not, an OBM is allocated and prepared with a SCRFR_RSP with BADREP status. Otherwise, the remote port 103(R) creates a route master 406 to track the progress of the full route registration and links it to a remote end-point incarnation proxy 409. If it was not possible to create a route master, then an OBM is allocated and prepared with a SCRFR_RSP with INSRES status to indicate that the remote port has insufficient resources to process the SCRFR.

In the case where the route master was created, the remote port 103(R) copies the SCRFR originator's full route handle and ep_info from the SCRFR message into the new route master. In the case where the new route master is the only route master at the remote port that describes this route, the remote port changes the route master's state to NRR and then notifies the remote end-point 102(R) by sending a New Route Responder (NRR) event, at step 917. In the case where the new route master duplicates an existing route master the two ports may be attempting to perform the full route registration protocol concurrently. This will be true if the existing route master is in the SCRFR_SENT state, in which case the UIDs of the route's end-points are compared and the port local to the end-point with the larger UID takes charge by running down the old route master and continuing as before by generating a NRR event at step 917. If the end-point that originated the SCRFR that is being processed has the larger end-point UID, then an OBM is allocated and used to prepare a SCRFR_RSP with DUPLICATE status. Then, if a SCRFR_RSP message was prepared, it is sent on the route management connection and the new route master is run down.

The (NRR) event informs the end-point referenced in the SCRFR that a new route 300 to the remote end-point incarnation specified by the remote end-point UID and remote end-point IID (incarnation identifier) is partly established through the path specified by the remote port 103(R). The remote end-point 102(R) either refuses the new route 300, or approves it and provides the port 103(R) with a responder ep_info structure, containing end-point information for the remote end-point 102(R), to transmit to the local end-point 102(L). In either case, the remote port 103(R) sends a SCRFR_RSP message, at step 925, to the local port 102(L) via the route management connection. Refusal causes the SCRFR_RSP status field to be set to REFUSED to indicate that the remote end-point refused the route, and the route's resources are run down. Approval updates the state of the route master 406 to SCRFR_RSP_SENT, sets the SCRFR_RSP status field to SUCCESS, and causes the message to convey the route's local and remote end-point incarnations (UID and IID), the responder's ep_info structure generated by the remote end-point, and a full route handle that can be used to direct messages from the local end-point 102(L) to the remote end-point 102(R) via the route 300.

Also in response to receipt of the SCRFR message at step 915, the local port 102(L) sends an ACK 920, either explicitly or piggybacked on the SCRFR_RSP, as part of the route management connection's protocol.

Receipt of the SCRFR_RSP with status field SUCCESS sent at step 925 causes the local port 103(L) to record the responder's route handle in the route master 406, change the state of the route master 406 to ACTIVE, and generate a Route Completed Originator (RCO) event at step 927, thus establishing the route at both the local port 103(L) and the local end-point 102(L), at step 930. The Route Completed Originator event informs the end-point that had previously approved the route 300 that the route is now complete and supplies the ep_info provided by the remote end-point. Because full route registration origination is flow controlled on each route management connection, a pending full route registration can now be started. Receipt of the SCRFR_RSP with a status field indicating anything other than SUCCESS causes the local port to run down the route master.

In response to receipt of the SCRFR_RSP at step 925, local port 102(L) sends an ACK 933 to remote port 102(R), either explicitly or piggybacked on another available message, as part of the route management connection's protocol. Receipt of that ACK causes the remote port to change the state of the route master 406 from SCRFR_RSP_SENT to ACTIVE and generate a Route Completed Responder (RCR) event at step 935, thus establishing the full route at the remote end-point 102(R) at step 940. More specifically, receipt, by the remote port 103(R), of a SCRFR_RSP ACK (at step 933) or receipt of the first message on the route 300 establishes the route from the local end-point 102(L) to the remote end-point 102(R), at step 940.

End-points 102(*) can remove the demand for new routes to a remote end-point by issuing a Derequest Remote End-Point message. A Deregister Full Route protocol is used to terminate a route. The Deregister Full Route process is triggered either by a Delete Route request by one of the route's end-points 102(*) or the failure of one of the route's end-point incarnations 410/411.

Route Set and Route Set Management Connection Establishment

As shown in FIG. 7, a route set 600 and a route set management connection 602 are established between end-point incarnations 410/411 at steps 725 and 730. Details of these steps are described with respect to FIGS. 10A, 10B and 10C, which are flowcharts showing exemplary steps performed in establishing a route set 600 and a route set management connection 602. Attributes of a route set 600 include a list of connections 601 between processes on the local end-point 102(L) and processes on the remote end-point 102(R) and a list of routes 300 to the remote end-point. A routing layer program running on an end-point 102(*) provides grouping of routes 300 into route sets 600, and uses a route set 600 to find candidate routes 300 to use for transmissions to the routing layer program running on the corresponding remote end-point 1013(*).

Route set and route set management connection establishment are driven by the New Route Originator (NRO) 910, New Route Responder (NRR) 917, Route Completed Originator (RCO) 927, and Route Completed Responder (RCR) 935 events that are generated by the ports 103(*) to the end-points 102(*) during full route registration. Thus, route set and route set management connection establishment are done by the end-points 102(*). Ports 103(*) are only involved in that they generate the events and communicate the ep_info data opaquely.

As shown in FIG. 10A, at step 1001, the routing layer 230 waits for a New Route Originator (NRO) or New Route Responder (NRR) event. When an NRO or NRR event is detected, a route proxy object 408 is created (at step 1005) that describes the new route 300. The route proxy 408 is described in detail below. If (at step 1007) a new route proxy 408 was not successfully created, then the new route 300 is refused. If a new route proxy 408 was created, then the UID and IID of the route proxy's remote end-point are used to find a corresponding route set among existing route sets 600(*) at step 1010. If a corresponding route set is found, then the new route proxy 408 is grouped with the other route proxies to the same end-point incarnation, by attempting to add the route proxy to the found route set 600 at step 1015. If no corresponding route set 600 presently exists, then the local end-point routing layer 230 attempts to create a route set 600 to the remote end-point 102(R) described in the new route event, at step 1012. If (at step 1017) a new route set 600 was successfully created, then the new route proxy is added to that route set at step 1015 as its first member, otherwise the new route 300 is refused, at step 1055. If the attempt to add the new route to the route set 600 (step 1015) failed (test at step 1020) (e.g. the route set was full), the new route proxy 408 is deleted and the new route is refused, at step 1055.

If the new route 300 was added in response to an NRR event (at step 1025), then the supplied route index is copied from the supplied ep_info, and the route set management connection (RTSMC) 602 is updated accordingly, at steps 1030 and 1035, respectively. An ep_info structure is then created for the remote end-point 102(*) at step 1040, and processing continues at step 1042, described below. If the new route 300 was added in response to an NRO event, then the route set management connection is updated at step 1027, and an ep_info structure is created for the remote end-point 102(*) at step 1029.

When two end-point incarnations 410/411 attempt to establish a route set management connection 602 between them simultaneously, only one RTSMC 602 should be created. When attempted on a single route 300, the SCRFR protocol resolves this conflict and generates a NRR event at only one end-point 102(*). However, when route set management connection establishment is attempted on two separate routes, NRR events will occur at both end-points 102(L)/102(R). To avoid creating two route set management connections 602, end-point UIDs are compared, and only the end-point 102(*) with the higher UID approves the route 300. Note that this UID comparison must be done with the same sense as that done in the SCRFR protocol in order to avoid refusing both attempts to create a route. Thus, at step 1042, if a duplicate route set management connection 602 exists in the requested state and the remote end-point UID has a value which is not greater than the local end-point UID, then the new route proxy 408 is deleted and the new route is refused, at step 1055.

Certain event conditions constitute a conflicting route set management connection 602 and thus cause an existing RTSMC 602 to be run down. The corresponding event is then processed as if the existing RTSMC were unknown. Those conditions include the following:

-   -   A NRR event whose originator and responder end-point UIDs and         IIDs match those of an existing route set 600, but whose         conn_id_requester does not match the RTSMC's outbound         connection_id (field 1408 in the routing header, described         below).

This implies that a stale RTSMC 602 exists at the responder.

-   -   A RCO event whose conn_id_acceptor does not match the RTSMCs         outbound connection_id. This implies that a stale RTSMC 602         exists at the originator.

When one end-point incarnation 410/411 attempts to establish a route set management connection 602 via two routes simultaneously, no special action is needed because both NRR events will reference the same conn_id_requester.

An end-point 102 can come up, go down, and then come back up with a new incarnation 410/411 fast enough so that the NRO and NRR events can arrive from the second (new) incarnation before those of the first incarnation. Therefore, both route sets 600 are initially allowed to be established, after which the stale one will fail naturally because it will be unable to deliver messages. When an end-point 102 has a choice of more than one route set 600 to a given end-point destination, preference is given to the last route set to become established. In addition, to accelerate the demise of the stale route set, when an end-point successfully sends a message on a RTSMC 602 and receives a response to that message, then the RTSMC is known to be functioning and any other route sets that exist to other incarnations of that end-point are stale and can be run down.

At step 1044, if there is no conflicting established route set management connection 602, then the state of the route proxy 408 is changed to indicate that the corresponding route 300 is in the process of being established, at step 1045. At step 1050, the ep_info structure that was constructed at step 1040 or 1029 (and mentioned in the description of FIG. 9) is passed to the port for transmission to the remote end-point and the route 300 is approved. Included in the ep_info structure are the inbound half route's identifier and the route's local route index (local_route_index), which is the index of the corresponding route proxy 408 in the local route set's route_proxies array (an attribute of the route set object 417, described below). The local route index value is sent to the remote end-point 102(R) during full route registration, where it is used by the remote end-point's routing layer 230 to identify the described route in the routing headers of subsequent messages sent on a particular route set 600.

If (at step 1044) there is a conflicting established route set management connection 602, then the new route proxy 408 is deleted at step 1046, the existing route set 600 is run down at step 1048, and an attempt is made to create a route proxy object 408 to describe the new route 300, at step 1005, and the above-described process is repeated.

As shown in FIG. 10B, at step 1060, the routing layer 230 waits for a Route Completed Originator (RCO) event or Route Completed Responder (RCR) event. When either an RCO or RCR event is detected, then the corresponding route proxy and route set objects 408/417 are located, at step 1062 as the states of these objects will be changed in response to the RCO/RCR event. If (at step 1063) a Route Completed Originator event was detected, then the supplied route index is recorded from the supplied ep_info structure at step 1066, and the state of the route proxy 408 is changed to active, at step 1067. The route set management connection 602 is then updated accordingly, at step 1068. At this point, the state of the route set management connection 602 may change in response to receiving the RCO event. In the situation wherein the RTSMC 602 is in the ‘requested’ state, and an RCO event is received in conjunction with an unknown connection ID, the state of the RTSMC is changed to established.

At step 1069, if there is a conflicting established route set management connection, then the route set is run down, at step 1080, and processing continues at step 1076; otherwise, processing continues at step 1070.

If a Route Completed Responder event was detected at step 1063, then the state of the route proxy is changed to active, at step 1064, and the route set management connection is then updated accordingly, at step 1065. Here, the state of the route set management connection 602 may change in response to receiving various events. In the situation wherein the RTSMC 602 is in the ‘accepted’ state, and an RCR event is received, the state of the RTSMC is changed to established.

At step 1070, if the route set management connection 602 was successfully established, then a check is made for connections 601 waiting for route sets and their connection establishment process is restarted, at step 1072. If the route set management connection 602 was not successfully established, then step 1072 is skipped.

At step 1076, a check is made to see if there are any outbound sequences (OBSs) waiting for an acceptable route and, if so, their route selection process is restarted (see 1325). At step 1078, the ep_info structure is passed to the port for inclusion in the SCRFR_RSP message, as indicated above with respect to FIG. 9.

As shown in FIG. 10C, at step 1080, routing layer 230 waits for a Route Deleted event, which is generated to remove a failed route 300 from a route set 600. When the network layer 240 delivers a route deleted event to the routing layer 230, it removes a failed route 300 from a route set 600. In addition, a route set 600 is automatically deleted as a result of events including the following:

-   -   no connection has existed on the route set 600 for a substantial         period, e.g., on the order of an hour. This indicates that there         is no demand for the route set;     -   no route has existed within the route set 600 for a substantial         period, e.g., on the order of an hour. This indicates the route         set cannot be supplied; or     -   no connection exists on the route set 600 and no route exists in         the route set.

When a Route Deleted event is detected, at step 1082, the corresponding route proxy and route set objects 408/417 are located. At step 1084, the route proxy 408 is removed from the route set 600 and deleted.

At step 1086, if the route set 600 is now empty, then (at step 1088) if the route set has no client connections, the route set is deleted, otherwise a timer is started which will delete the route set if no routes 300 are added before the timer expires. If the route set 600 is not empty at step 1086, then Route Deleted event processing terminates, at step 1090.

Application Connection Establishment

As shown in FIG. 7, a connection 601 between local and remote end-point applications 101(*) is established at step 735. These application connections are derived from a routing layer connection base class. The aspects of the application connection described in this section are actually those provided by the routing layer connection, and thus only the routing layer connection is described subsequently.

As explained above, route set 600 also provides a route set management connection 602 that is used to manage the routing layer connections 601 established on the route set. This includes the communication of Connect Request and Connect Accept messages used to establish the connections as well as the Connect Abort message used to destroy the connections.

During routing layer connection establishment, routing layer 230 assigns connection IDs. Once the connection is established, the request layer copies the connection ID into the routing header of each outbound message, and dispatches incoming messages to their proper client based on the connection ID in the received routing header. The routing header (1400) is described in detail with respect to FIG. 14, below. Each routing layer connection 601 is established on a route set 600. The route set 600 defines the set of available routes 300 that can be used by the connection 601.

Routing layer 230 relies on the transport layer 220 to send and receive messages on a route set management connection 602. In this role, the routing layer 230 is merely a client making use of transport layer functionality. Thus, the transport layer 220 must allow each connection 601 to have a different client.

FIG. 11 is a flowchart illustrating exemplary steps performed in handling a request to establish a connection 601 between end-point applications 101(*). As described in detail in the present section, the routing layer 230 establishes an application connection 601 with a three-way handshake beginning with a Connect Request message on the route set management connection 602. Receipt of a Connect Accept message completes the handshake at the requester and receipt of either the connect accept ACK or the first message on the routing layer connection 601 completes the handshake at the acceptor.

As shown in FIG. 11, at step 1105, when routing layer 230 receives a request to establish a connection 601 to a remote end-point 102(*), the routing layer must first find an established route set 600 to that remote end-point. This is accomplished by a standard software lookup procedure.

At step 1110, if no established route set 600 exists, the routing layer will attempt to create a route set 600 by issuing a request remote end-point message to each of the network layer's local ports at step 1120. This triggers an autonomous full route registration process in the network layer's local port, as described above with respect to FIG. 9. From this point on, route establishment and route set establishment are driven by events generated by the network layer 240.

At step 1125, a routing layer routine waits for a route set 600 to the requested remote end-point 418 to be established. After the route set 600 and a route set management connection 602 are established to the requested remote end-point 418, then an OBM is allocated by the client wanting to send the message, at step 1130.

At step 1110, if an established route set 600 exists to the remote end-point 102(R), then an OBM (outbound message object) is allocated by the client wanting to send the message, at step 1130, and processing continues as described below with respect to step 1135.

In the present case, routing layer 230 is the client. An OBM is an object that is shared between the routing layer and its client, and used by the routing layer's client to describe a message that is to be sent by the routing layer. The OBM is passed to the routing layer in various functions that select routes and send messages. In the present case, the routing layer allocates an OBM so that it can send a Connect Request message on a route set management connection. Once the OBM is allocated, the routing layer calls a Make Connect Request virtual function, at step 1135, to allow the routing layer's client to add its information to the Connect Request message being constructed within the OBM, at step 1140. The Connect Request message is then sent on the route set management connection 602 to the remote end-point 102(R), at step 1145.

When the Connect Request message is received by routing layer 230 at the remote end-point 102(R), the routing layer delivers it to the transport layer entity to which the message is addressed, at step 1150. The transport (or a higher layer) then determines whether or not a matching Connect Accept message is pending, at step 1155. If there is a pending Connect Accept, then an OBM is allocated by the routing layer, at step 1165, and the routing layer calls a MakeConnectRequest virtual function to allow the routing layer's client to add its information to the Connect Accept message, at step 1170. The Connect Accept message is then sent on the route set management connection 602, at step 1175.

If no matching Connect Accept is pending when the Connect Request message arrives at the receiver at the remote end-point 102(R), then the routing layer allocates a OBM and responds with a Connect Reject message, at step 1160.

When the Connect Accept message is received, at step 1180, the routing layer application connection 601 becomes established at the requestor, and the routing layer's client is notified with a ConnectRequestDone virtual function associated with the connection.

The routing layer application connection 601 becomes established at the acceptor when the ACK to the Connect Accept message is received (step 1185) or the first message is received on the new connection by the remote end-point 102(R) (step 1190). A ConnectAcceptDone virtual function then notifies the routing layer's client that the connection has been successfully established.

Route Selection

Once a connection, including an application connection 601 or a route set management connection 602, is established between end-points 102(*), the clients of that connection can use it to exchange messages reliably. Each message (e.g., a Connect Request message) is preferably sent via its initial transmission, and retry transmissions are used by the transport layer to assure that messages are delivered reliably in the same manner employed by TCP. In the present system, the transport layer 220 requests that the routing layer 230 send these transmissions and the routing layer selects a route 300 for each transmission and then sends the transmissions via the selected route.

The present route selection method is based on the notion of acceptable routes. An acceptable route is one that meets all of the following constraints:

-   -   the outbound half route 500 is enabled and in the active state;     -   the outbound half route's transmission rate is below its         transmission rate limit; and     -   the outbound half route's local port 103(L) has the resources         required to send a transmission, including an available outbound         sequence (OBS) object, which is the primary object describing a         transmission to be sent.

As shown in FIG. 7, at step 740, a route 300 between two end-points 102(*) is selected in response to a route selection request. Route selection is done each time the routing layer 230 sends a transmission, which may be the initial transmission of a message, a retry transmission of a message, or an ACK. The route selection process is described in detail with respect to FIGS. 12A and 12B. Before making a route selection request, the routing layer client must first allocate and construct an OBM that describes the outbound message to send, and this OBM must be available for queueing in the case where not acceptable route is available.

FIG. 12A is flowchart illustrating an algorithm comprising exemplary high-level steps performed in selecting a route 300 for an initial transmission. As shown in FIG. 12A, when selecting a route for an initial transmission, the last route 300 used within the route set 600 is selected if it is still acceptable (step 1110) and has been used for fewer than some small number of consecutive transmissions CTmax (step 1215). The value of CTmax is selected by the implementer, but is typically approximately 5, and tends to amortize the selection computation over that number of transmissions.

At step 1225, a determination is made as to whether there are any acceptable routes 300 in the route set 600. If no acceptable routes are found, then at step 1230, the OBM supplied in the route selection request is queued in the routing layer. If there are a plurality of acceptable routes 300 in the route set 600 that are equally underutilized (step 1235), then a route is selected from these equally underutilized acceptable routes at step 1240, that is next in route set order starting with the last route used; otherwise, the least utilized acceptable route from the route set 600 is selected at step 1245.

FIG. 12B is flowchart illustrating exemplary high-level steps performed in selecting a route 300 for a retry transmission. As shown in FIG. 12B, at step 1250, a determination is made as to whether there are any acceptable routes 300 in the route set. If acceptable routes exist, then, at step 1265, the route 300 is selected which is the next acceptable route in route set route order relative to the route used for the previous transmission of the message being retried. This procedure assures that for any given message, all acceptable routes are tried before any are retried. If no acceptable routes 300 exist in the route set 600, then at step 1260, the OBM supplied in the route selection request is queued within the routing layer 230.

When a route's state changes such that it may now be acceptable, the route selection algorithm of FIG. 12A/B is executed, and queued route selection requests may complete asynchronously. The passing of time will automatically lower a route's transmission rate below its transmission rate limit and when the other constraints are met, then transmissions will be sent at the route's transmission rate limit. A method of controlling the transmission rate for a given half route 500 is described below with respect to FIG. 17.

Queueing

FIG. 13 is a diagram showing exemplary steps and queues used in queueing route selection requests. As shown in FIG. 13, when a route selection request is received by the routing layer 230 (FIG. 12A, step 1230, or FIG. 12B, step 1260), a determination is made as to whether any acceptable routes 300 are presently available within the specified route set 600, at step 1305. If an acceptable route 300 exists, then route selection proceeds in accordance with the steps shown in either FIG. 12A or FIG. 12B. When no acceptable routes 300 are available within the specified route set 600, the OBMs supplied in the requests to select routes are queued within the routing layer 230, at step 1315. When a route's state changes such that it may now be acceptable, the route selection algorithm described with respect to FIG. 12A/11B is executed and queued route selection requests may complete asynchronously. Because an acceptable route must have a transmission rate that is below its transmission rate limit, the routing layer is able to control transmission rate using its queueing mechanism. This feature is used to back off transmission rate when a route becomes congested.

Each route selection request issued to the routing layer 230 is described by an outbound message (OBM) object that the routing layer may queue. When no acceptable route is available, the request's OBM is placed on one of the following prioritized FIFO queues:

1. A route_proxy.send_specific_route queue 1320 is used to queue requests for which the route has already been determined. Although no route selection is done in this case, route status must be acceptable and an outbound sequence (OBS) must be allocated; these are effectively route selection functions. These requests, for which the route has already been determined, are given highest priority because the delivery of an End_MSE (end multi-sequence exchange) frees resources and because the local resource allocation is constrained.

2. A route_set.send-retry_sfe queue 1321 is used to queue route selection requests for retry SFEs (single frame exchanges). These requests are given second highest priority because they indicate that connection message delivery is probably stalled at the remote end-point.

3. A route_set.send_initial-sfe queue 1322 is used to queue route selection requests for sends of initial SFEs. These requests are given third highest priority because they will stall the connection's message transmission unless satisfied.

4. A route_set.send_start_mse queue 1323 is used to queue route selection requests for Start_MSEs (start multi-sequence exchanges).

When a route's state changes such that it may now be acceptable (step 1325), the route selection algorithm of FIG. 11A/11B is executed with the OBMs at the heads of these queues for the corresponding route proxy 408 and route set 600 in the priority order listed above, at step 1330. If the route selection algorithm indicates that a route 300 is acceptable for an OBM, the OBM is dequeued. Then an OBS is allocated from the route's local port 103(L), and after the OBS is prepared it is transmitted via the local port 103(L).

When the route selection algorithm determines that a route 300 is unacceptable because no OBS is available on the route's local port 102(L), the corresponding route set 600 is queued to the local port. When an OBS becomes available on that local port, route sets 600(*) are dequeued and their route selection algorithm is run until one consumes the available OBS. Because a route set 600 may need to be queued to several local ports 103(*) simultaneously, the unacceptable route's route proxy 408, instead of the route set, is queued to the local port 103(L). Instead of dequeueing a route set 600, a route proxy 408 is dequeued along with all other route proxies in that route's route set 600 that use the same local port 103(L).

Route Order

The two end-points 102(*) of a route set 600 each determine an order for the routes 300 in the route set. There are several reasons for route ordering. First, when a route 300 becomes unacceptable, it is desirable to quickly select a next route from the same route set that is largely independent of the old route. Since route independence is relatively static, a next route can be predetermined for each route. In addition, when a series of two or more routes becomes unacceptable, it is desirable to quickly select a next route from the same route set that is largely independent of each route in the series of unacceptable routes. All routes of a route set may be placed in a circular queue to facilitate this route selection.

Each end-point 102(*) determines route order independently, using locally available information, and then communicates that order to the corresponding remote end-point. The remotely determined route order is used to select the next route for transmission because it may include information about route independence that is not available locally. When a new route 300 is added to a route set 600 or removed from a route set, the route set's route order changes and the new route order is communicated via a route set management connection message. While changes in route set order may cause transient anomalies in route selection, these anomalies do not cause incorrect network behavior and should disappear quickly.

Route Order Report (ROR) messages are sent on the route set management connection 602 to communicate a route set's locally determined route order to the remote end-point 102(R). The ROR is a one-way sequenced message that includes a count field indicating the number of routes 300 in a route set 600 and an array containing an ordered list of the sender's remote_route_index values of the routes in the sender's local route set route order. The remote-route-index comprises indicia of the remote route proxy 408 for a given route 300 in the remote route set object's route_proxies array.

Route order determination is not architecturally constrained, because it does not affect correctness of operation. However, route order can have a significant effect on performance. In general, successive routes 300 in a route set 600 should share as few common components or resources as possible. This is desirable in error recovery, to maximize chances of a retry succeeding. It is also desirable for load balancing, where the goal is to spread the load as uniformly as possible across the available components. The manner in which the route ordering is determined is described in a related application.

Routing Header

All routing layer transmissions include a routing header 1400. The routing layer 230 uses information in the routing header 1400 to monitor transmissions on each half route 500 to determine the half route's quality, by determining whether a half route is delivering transmissions promptly, has lost recent transmissions, or is experiencing delay. Inbound half route quality is computed at the receiving routing layer and converted to a transmission rate limit, which is communicated back to the transmitting routing layer in the routing header where it is used by a route selection algorithm (e.g., the algorithm of FIGS. 12A/B) to choose the best route 300 of a route set 600 for sending future transmissions. Finally, the routing header contains the routing layer connection's connection_id that was discussed in reference to FIG. 11.

In an alternative embodiment, the routing layer places information into the routing header of each transmission sent on a route set that describes multiple (or even all) routes of that route set. However, the result is expensive both in transmission header efficiency and processing time to send and receive the transmission. In an exemplary embodiment, the routing layer 230 places monitoring information that describes a single route of the route set in the routing header 1400 of each transmission sent on that route set 600, and in the steady state, the single route described is rotated among the routes of that route set in a round-robin fashion. Thus, the described route may differ from the route used to send the transmission.

For a given transmission, the route 300 described in the routing header 1400 is called the described route (DR). In contrast, the route 300 on which the transmission is sent and received is called the receive route (RR). Routing header 1400 includes the following information about the described route:

-   -   the DR's index, which uniquely identifies the DR to the         receiver;     -   the RSN of the last transmission sent on the outbound half DR;     -   the time since the last transmission was sent on the outbound         half DR;     -   information that signals that the transmission rate limit (TRL)         should be reset on the receiver's inbound half DR; and     -   the computed TRL for the transmitter's inbound half DR.

In an exemplary embodiment, the routing header 1400 contains the following fields, which are transmitted in the order listed. All fields except the connection ID describe a route in the route set known as the described route or DR. These fields are organized within an exemplary routing header as shown in FIG. 14, and described below. dr_index key This 2-bit field (‘key’ 1401 in FIG. 14) is a key that is used to validate the dr_index field at the receiver. Its value is determined by the receiver and is communicated to the transmitter during full route registration. When a transmission is received in which this value does not match the expected value then the described route information in the transmission is ignored. dr_tre This 2-bit field (‘tre’ 1402 in FIG. 14) specifies the transmitter's route epoch for the outbound half DR. The receiver compares this value to the last dr_tre value received for the inbound half DR. If it differs, the receiver resets the inbound half DR's transmit rate limit to one transmission per E_D_TOV (defined in FC-FLA V2.7). dr_rre This 2-bit field (‘rre’ 1403 in FIG. 14) is a copy of the last dr_tre received by the transmitter for its inbound half DR. When this field equals the receiver's outbound half DR route epoch, then the local end-point knows that the remote end-point has received its last route epoch update for the DR. dr_trlc This 8-bit field 1404 is a compressed form of the transmit rate Limit (TRL) computed by the transmitter for its inbound half DR. This value is used by the receiver to limit the transmit rate of its outbound half DR. dr_itt This field 1405 contains an 11-bit unsigned integer representing the number of milliseconds that have elapsed since the last transmission on the transmitter's outbound half DR. If the elapsed time exceeds 2047 milliseconds, then this field contains a value of 2047. dr_index This field 1406 contains a 5-bit unsigned integer representing the index of the inbound half DR at the receiver. This index is supplied to the transmitter during full route registration. The receiver uses this value to determine which inbound half route is being described. dr_rsn This field 1407 contains a 32-bit unsigned integer representing the routing sequence number of the transmitters outbound half DR. This value is the number of transmissions sent on the route since it was established, modulo 2³². connection_id This field 1408 contains the 32-bit outbound connection ID of the connection on which the message is being sent. The receiver uses this value as an inbound connection ID to find the destination connection.

End-points 102(*) monitor the quality of each inbound half route 500 using data supplied by the remote end-point 102(R) via the routing header 1400. The receiving end-point combines all said data into a single metric called the transmission rate limit (TRL). The TRL is indicative of whether a half route is delivering transmissions promptly, has lost recent transmissions, or is experiencing delay. Each TRL computed by the receiving end-point is fed back to the transmitting end-point via a field in the routing header 1400, which, in one embodiment, is sent in compressed form as the dr_trlc field 1404. At the transmitter, the TRL is used to place an upper bound on the rate of transmissions issued on the outbound half route 500 and by a route selection algorithm (e.g., the algorithm of FIGS. 12A/B) to choose the best route 300 of a route set 600 for sending future transmissions. In this capacity, the ratio TR/TRL is used to determine whether a route is acceptable and as a measure of route utilization. These processes are described below in detail with respect to FIGS. 12 and 17.

Send Processing

FIGS. 15A and 15B are diagrams illustrating exemplary routing layer processing performed in sending transmissions. The routing layer client sends a transmission via a routing layer connection in two phases. First, the routing layer client selects a route 300, and then the routing layer client sends the transmission on that route. The routing layer provides separate procedures for selecting and sending the initial transmission of a message, shown in FIG. 15A, and for selecting and sending retry transmissions of a message, shown in FIG. 15B.

As shown in FIG. 15A, the route selection phase for an initial transmission begins at step 1502, in which the routing layer client supplies a reference to the routing layer connection and a outbound message (OBM) structure that describes the message to be transmitted. At step 1505, the routing layer connection state is validated as suitable for sending transmissions, i.e., the connection state must be established. At step 1508, a route is selected for an initial transmission as described in FIG. 12A, queueing for an acceptable route if necessary as described in FIG. 13. Once route selection completes, the routing layer must check to see that a route was indeed selected at step 1511, because it is possible, for example, that the route set failed while the OBM was queued for an acceptable route. In the case where a route was selected, an outbound sequence (OBS) structure must be allocated from the route's local port for describing the transmission at step 1514, and linked to the OBM. The selected route is recorded in that OBS at step 1517, and the selected route is recorded in the OBM as the last route tried for transmissions of the message described therein at step 1520. Finally, at step 1523, the routing layers estimate of the selected route's outbound half route transmission rate is updated and control is returned to the routing layer client at step 1526.

Before the routing layer client requests that the routing layer send the initial transmission at step 1529, the routing layer client may update its connection state. For example, the routing layer client might start a timer that triggers a retry transmission if the initial transmission is not acknowledged before the timer expires. When the routing layer client does request that the initial transmission be sent, the routing layer starts by initializing the OBS that was previously linked to the OBM at step 1514. At step 1532, the route's route master handle is inserted in the OBS so that when the OBS is passed into the port layer then the port layer can rapidly find the outbound route. At step 1535, a scatter-gather list (SGL) is copied to the OBS that describes the OBM buffer that holds the transmission's routing, transport, and request layer headers plus the application payload buffer. Thus, the port can transmit all of this data directly from the end-point's buffers without having to first copy it into a contiguous buffer. At step 1538, the described route (DR) is selected, and the routing header described route fields are filled in using data from the selected described route. Next, the routing header's connection ID 1408 is filled in using data from the routing layer connection, at step 1541. The receive route (RR) is then updated by incrementing its RSN at step 1544. Next, the network layer 240 is used to send the transmission via the selected route's local port 103(L), at step 1547. Once the network layer completes its attempt to send the transmission, the status is checked. In the case where the network layer was unable to successfully send the transmission, then the receive route (RR) is updated at step 1550 to indicate that it is seriously congested. Regardless of the send completion status, the OBS is unlinked from the OBM and de-allocated at step 1553 and control is returned to the routing layer client.

As shown in FIG. 15B, the route selection phase for a retry transmission begins at step 1564 in which the routing layer client supplies a reference to the routing layer connection and a outbound message (OBM) structure that describes the message to be retransmitted. At step 1567, the routing layer connection state is validated as suitable for sending transmissions, i.e., the connection state must be established. Because this is a retry transmission, it is very likely that the previous transmission of this message failed, thus at step 1570 the TRL of the half route indicated by the OBM's last route tried (set in step 1520) is decreased and the half route's significant event flag is set. At step 1573, a route is selected for a retry transmission as described in FIG. 12B, queueing for an acceptable route if necessary as described in FIG. 13. Once route selection completes the route selection phase for a retry, transmission continues, and the steps shown in bracket 1580 are performed exactly as corresponding steps 1511-1523 (described with respect to FIG. 15A) for an initial transmission.

The send transmission phase for a retry transmission begins at step 1587. The routing layer starts by initializing the last OBS that was linked to the OBM. At step 1590, the route's route master handle is inserted in the OBS so that when the OBS is passed into the port layer then the port layer can rapidly find the outbound route. At step 1593, a scatter-gather list (SGL) is copied to the OBS that describes the buffer that holds the transmission's routing, transport, and request layer headers plus the application payload buffer. In contrast to the initial transmission, the SGL for the retry transmission describes a buffer for the headers that is in the OBS instead of the OBM, and the request and transport headers are copied from the OBM to the OBS in step 1596. This allows the routing layer to modify the routing header without risking modifying said header of the initial transmission of the message which may not be complete at the time of the retransmission. Once the header copy completes the send transmission phase for a retry, transmission continues, and the steps shown in bracket 1598 are performed exactly as corresponding steps 1538-1556 (described with respect to FIG. 15A) for an initial transmission.

Receive Processing

FIG. 16 is a diagram illustrating exemplary routing layer processing performed in receiving transmissions. As shown in FIG. 16, the routing layer performs the following exemplary steps with respect to each received transmission. Initially, at step 1605, context is established from a pool buffer descriptor, which includes indicia of the route over which the transmission was received, and information supplied by the receiving port about how the transmission was received. In an exemplary embodiment of the present system, communicating applications 101(*) create buffer pools, containing pool buffers, to directly receive messages bound for them. When an application 101 requests or accepts a connection 601, it specifies the buffer pool into which messages received on that connection should be stored. In an exemplary embodiment, end-point memory 421 is used for the buffer pools, as applications 101 are executed in that memory space. However, when a pool buffer (i.e., a buffer within the buffer pool) is posted for receive, a description of that buffer is passed to the associated port 103 and that memory becomes shared between the end-point 102 and the port until the pool buffer is consumed. This allows the port to receive a transmission directly into the pool buffer without the need to copy the data from the port memory 420 to end-point memory 421.

Given the route over which the transmission was actually received, the routing layer is able to determine the corresponding RR and the route set 600 at step 1605. The DR is determined at step 1608 from the route set itself and dr_index 1406 and dr_index key 1401 (in the routing header). At step 1610, the route set, DR, RR, and the routing header data are used to compute a new transmission rate limit (TRL) for the inbound half DR. Next, at step 1615, the transmission rate limit (TRL) for the outbound half DR is updated using the dr_trlc 1404 that was fed back from the other end-point via the routing header. Since updating this TRL may have caused the DR to become acceptable, a check is made for OBMs queued for route selection. Then, at step 1618, the reception rate estimate, round-trip-time estimate, inbound TRL, and expected RSN for RR are updated. The destination routing layer connection whose inbound connection ID matches connection_id 1408 (in the routing header) is then located, at step 1620. At step 1623, if the destination routing layer connection is in the accepted state then transition the routing layer connection to the established state and run down any existing route sets to other incarnations of the same remote end-point. At step 1625, if the destination routing layer connection state is suitable for receiving transmissions (e.g., established), then the transmission is delivered to the routing layer connection's client at step 1630 by passing the pool buffer descriptor to it; otherwise, the transmission is discarded, at step 1635 by returning the pool buffer descriptor to its buffer pool.

Significant Events

Most events that cause changes in half route quality metrics are known as significant events. When a significant event occurs, it becomes desirable to describe the affected route 300 to the remote end-point promptly. Normally, the routes 300 in a route set 600 are described in round-robin order, but following a significant event, the corresponding route is given a higher priority for being described. This is done with a significant_event flag (an attribute of the route proxy object 408) that is set when a significant event occurs on that route 300 and which is cleared when the route is described.

The following events are considered to be significant events:

-   -   The routing layer client indicates that an ACK response timeout         occurred on a route 300. In response, the outbound half route's         transmission rate limit (TRL) is reduced by half and its         significant_event flag is set. Because the ACK may have been         sent on any route, the routing layer 230 cannot conclude with         certainty that the indicated half route 500 has lost or delayed         any transmissions. By changing the outbound half route's TRL,         the local end-point 102(L) is immediately discouraged from using         that route further, but when the remote end-point 102(R)         provides an updated TRL, this change will be dismissed, since         the remote end-point has more definitive quality data about the         outbound half route 500.     -   Any time the outbound half route 500 selected for an initial         transmission differs from the route set's last_rtp. The value         for last_rtp indicates the route proxy 408 of the last route 300         that was used to send a transmission on this route set 600; this         value is included as a field in the route set object 417,         described in detail below. In response, the significant_event         flags are set for the selected route 300 and last_rtp, and         last_rtp is updated to be the selected route. This is done to         describe changes that motivated selecting a new route 300 and to         assure that routes describe themselves frequently.     -   Any time the outbound half route 500 selected for a retry         transmission differs from the route set's last_rtp. In response,         the significant_event flag is set for last_rtp and last_rtp is         updated to be the selected route 300. The significant event flag         is not set for the selected route so that the route used for the         previous try will be described as soon as possible.     -   Any time a half-route's local port changes state, the         significant_event flags for all associated outbound half routes         are set. In addition, when a local port changes state from         link-up to link-down, the TRLs for all associated half-routes         are set to zero, and when a local port changes state from         link-down to link-up, the TRLs for all associated half routes         are set to their default values.         Route Master/Proxy Objects

The combination of the route master object 406 and the route proxy object 408 describes the state of a route 300 from the local end-point incarnation 411 to a remote end-point incarnation 410. The route master object 406 describes the portion of the route that is important to the local port 103(L) including the associated route management connection 603 [which implies the remote port 103(R)], the full route handle, references to the local and remote end-point incarnations 411/410, state, and a reference to the corresponding route proxy 408.

The route proxy object 408 describes the portion of the route that is important to the end-point 102 including a reference to the route's route set 600, the route's index within the route set, the route's order within the route set, the routing sequence number for the outbound half route, the transmission rate limit for the outbound half route, information used to measure the quality of the inbound half route, state, and a reference to the corresponding route master object 406.

Route Master Object

Each port 103 maintains route master objects 406, each of which describes a relationship between a local end-point incarnation 411, a remote end-point incarnation 410, a remote port 103(R), and (by implication) the local port 103(L). A route master object 406 is created whenever a port 103(*) discovers a unique route 300 between a requested remote end-point 415 and one of the requested remote end-point's local end-point incarnation proxies 412. This may occur because either a new partial route is discovered to an existing requested remote end-point 415, or a local end-point incarnation 411 issues a request remote end-point message to an end-point 102 for which a partial route already exists.

A route master object 406 is also created whenever a port 103 receives a Register Full Routes (SCRFR) message. A route master object 406 is deleted whenever any of its references become invalid, which may occur in the following cases:

-   -   The associated path becomes invalid because either the remote         port 103(R) is implicitly logged out (i.e., becomes not usable)         or the associated route management connection 603 fails, or a         SCRPR arrived on that path that omitted the route master's         remote end-point incarnation 410.     -   The associated local end-point incarnation proxy 412 is deleted         because the local end-point 102(L) either deregistered or         reregistered.     -   The associated route proxy 408 is explicitly deleted.

A route master object 406 is also deleted whenever a route 300 is deregistered via SCDFR. Finally, incomplete route master objects 406 are deleted when SCRFR is rejected via a SCRFR_RSP and when the new route event is refused.

Exemplary attributes associated with a route master object 406 include the following: full_route_handle The full route handle received in the SCRFR or SCRFR_RSP message used to establish this route 300. It is used to direct messages to the remote end-point 102(R) via the route. local_end_point_incarnation_proxy Handle of the associated local end-point incarnation proxy 412 or pseudo local end-point 402 (the null handle is used to reference the pseudo local end-point). remote_end_point_incarnation_proxy Handle of the associated remote end-point incarnation proxy 409 or pseudo remote end-point 403. remote_port Handle of the associated remote port object 401. route_management_connection Handle of the associated route management connection object 603. route_proxy Handle of the associated route proxy 408. state state of the route master object 406. Values for the route master state include: Unknown: Implies that the route master does not exist. NewRouteOrig: Implies that the route master has generated a new_route_originator event and is awaiting a response. NewRouteResp: Implies that the route master has generated a new_route_responder event and is awaiting a response. SCRFR_Sent: Implies that the route master has sent a SCRFR message and is awaiting a response. SCRFR_RSP_Sent: Implies that the route master has sent a SCRFR_RSP message and is awaiting its ACK. Active implies the normal operational state of the route master. RunDown: Implies that the route master is in the process of being deleted, but still exists because references to it still exist. On explicit creation, the value of this attribute is set to NewRouteOrig. On creation due to receiving a SCRFR message, the value of this attribute is set to NewRouteResp. Route Proxy Object

The route proxy object 408 is a structure that describes a single route 300. It is a proxy of the route master object 406. The route proxy object 408 describes the state of a route 300 from the local end-point 102 to the remote end-point 102 through a specific path. Attributes of the route proxy 408 include its route_set, rtp_handle, rtm_handle, local_port_proxy, and state. These attributes are involved in route selection and monitoring. Route proxy objects 408 are created and deleted in response to events (new route originator, new route responder, route deleted) generated by route masters 406 on local ports 103(L). Route proxy objects 408 are also deleted when the local port 103(L) fails.

Exemplary attributes associated with a route proxy object 408 include the following: rtp_handle Handle that the route master 406 uses to identify the route proxy object 408. expected_rsn The expected value of the routing sequence number that will be received in the next transmission received on this route that describes itself. highest_described_rsn The highest RSN received that described this route. It is used to calculate transmission delay likelihood. On creation, this value is set to zero. subsequently, it is updated each time a transmission is received that describes this route in which the described RSN exceeds this value. inbound_trlc Transmission rate limit of the inbound half 500(I) of this route 300 in compressed form. This value is computed locally and commun- icated to the remote end-point 102(R) where it is used to limit the transmission rate of the corresponding outbound half route 500(O). last_receive_time The time when the last transmission was re- ceived on this route. Used to calculate recep- tion_rate_est. last_send_time The time when the last transmission was sent on this route. Used for calculating transmission_rate_est. local_port_proxy The local port proxy 413 for the port 103 that contains this route proxy's route master 406. local_route_epoch This route proxy's route epoch. The route epoch is incremented to cause the remote end- point 102(R) to reset its TRL computation for the inbound half route 500(I). local_route_index Index of this route proxy 408 in the local route set's route_proxies array. This value is sent to the remote end-point 102(R) during full route registration. The remote end-point uses it to identify this route 300 in route headers 1400 that it transmits. local_route_index_key Opaque key used to validate the local_route_index. This value is sent to the remote end-point 103(R) during full route registration and a copy is received in the rout- ing header 1400 when the transmission de- scribes the route 300. When a received trans- mission's local_route_index_key does not match this attribute, then the described route information is ignored. next_described_route The next route 300 in the route set 600 to de- scribe in the routing header 1400 of a trans- mission sent on this route. next_route The next route in the route set's route order. It is a forward link in the circular list of all routes 300 of the route set 600. one_shot_flag Used when transmission_rate_est > de- compressed (outbound_trlc) to make the route acceptable for one transmission. outbound_trlc Transmission rate limit of the outbound half 500(O) of this route 300 in compressed form. it is used to determine whether a route is accept- able during route selection. On creation, this value is set to SCR_TRLC_DEFAULT. Subsequently, it is updated when a received transmission describes this route. It may also be updated when the local end-point 103(L) suspects that the outbound half route's quality is low. reception_iat_est Estimate of the mean transmission receive inter-arrival time of the outbound half 500(O) of this route 300. On creation, this value is set to scr_irr_scale. Subsequently, it is updated each time a transmission is received on this route. remote_route_epoch The last value of transmitter_route_epoch described for the inbound half 500(I) of this route 300. This value is sent in the routing header 1400 of each transmission that describes this route. On creation, this value is set to zero. Subsequently, it is updated when a received transmission describes this route. remote_route_index Index of the remote route proxy 408 for this route 300 in the remote route set's route_ proxies array. It is copied to the routing header 1400 in each transmission that describes this route. remote_route_index_key Opaque key used to validate the remote_route_index. This value is received from the remote end-point 102(R) during full route registration and is transmitted in the rout- ing header 1400 with the remote_route_(—) index. round_trip_time_est Estimate of the round-trip-time for this route 300. Its value is a number of milliseconds * 256 expressed as an unsigned 32-bit integer. On creation, this value is set to 256 (1 milli- second). route_set The route set 600 to which the route proxy 408 belongs. On creation, its value is set to null. However, every route proxy 408 is added to a route set 600 immediately after creation and only removed from a route set immediately prior to run-down. rsn The route proxy's routing sequence number. Its value is the count of transmissions originated at this end of the route expressed as an unsigned 32-bit integer. rtm_handle Handle that identifies the route master object 406 that corresponds to this route proxy 408 within the scope of a local port 103(L). Its value is supplied by the route master 406 when the route proxy 408 is created and cannot be modified subsequently. send_specific_route_queue Queue of obms waiting for this route 300 to send a transmission. Used when an allocateobs operation requires a specific route but that route is not acceptable. significant_event A binary flag indicating that the route 300 has experienced a significant event since the route was described. state State of the route proxy object 408. timer A general-purpose timer for the route used for a variety of purposes. transmission_iat_est Estimate of the mean transmission send inter- arrival time of the outbound half 500 of this route. On creation, this value is set to scr_irr_(—) scale. Subsequently, it is updated each time a route selection algorithm selects this route for a transmission. transmission_rate_est Estimate of the transmission rate of the out- bound half 500(O) of this route. On creation, this value is set to zero. subsequently, it is updated each time the route selection algorithm selects this route for a transmission. transmissions_received The count of transmissions received at this end of the route expressed as an unsigned 32-bit integer. This attribute is used as a performance counter, i.e., expected_rsn - 1 - trans- missions_received gives the total transmissions lost on this route. Route Set Object

A route set object 417 associates the routes 300 from a local end-point incarnation 411 to a remote end-point incarnation 410. Attributes of the route set object 417 include a list of routes 300, a list of clients (connections) 601, and a remote end-point UID and IID (the local end-point 102(L) is implicit).

A route set 600 groups the complete set of route proxies 408 that correspond to approved routes 300 between this local end-point incarnation 411 and a remote end-point incarnation 410. The route set object 417 is used to manage the creation and deletion of routes 300 between a pair of end-points 102(*) and groups the routes that exist between those end-points.

As indicated above, a route set 600 is created when a route proxy 408 is created to a remote end-point incarnation 410 for which no route set yet exists. A route set 600 is deleted immediately when it has neither route proxies nor client connections. Stale route sets 600 are also deleted using policies based on an empty timeout (SCR_E_TOV) and a demand timeout (SCR_D_TOV).

When a route set 600 has clients but no route proxies 408 for a continuous period (SCR_E_TOV) of time, it will enter the rundown state and issue Derequest Remote End-Point commands to each local port 103(L) to stop the creation of new routes 300. In an exemplary embodiment, the value for SCR_E_TOV is 600,000 (i.e., ten minutes). The route set object 417 then generates a route_set_failed event to each of its client connections 601. As each client connection aborts, it will remove its route set reference and thus allow the route set 600 to be deleted. SCR_E_TOV is used to control local policy that recovers resources from empty route sets. Specifically, when a route set 600 has no routes for SCR_E_TOV continuous milliseconds, it may be forced into rundown.

Exemplary attributes associated with a route set object 417 include the following: client_list Unordered list of client connections 601 that use this route set 600. demand_timer Timer to time SCR_D_TOV. On creation, this timer is started. It is cancelled when client_list transitions from empty to nonempty. It is restarted when client_list transitions from nonempty to empty. establishment_time Time when the route set's RTSMC 602 entered the established state. This is used to determine the most recently established route set 600 when selecting between multiple route sets to the same remote end-point 102(R). empty_timer Timer to time SCR_E_TOV. On route set creation, this timer is started. It is cancelled when route_proxies transitions from empty to nonempty. It is restarted when route_proxies transitions from nonempty to empty. last_rtp Indicates the route proxy 408 of the last route 300 that was used to send a transmission on this route set 600. On creation, this value is set to NULL. When the first route is added, it becomes a reference to that route. When the last route is removed then it becomes NULL. last_rtp_count Count of the consecutive route set transmissions sent on route last_rtp. On creation, this value is set to zero. The route selection process zeros it each time it changes last_rip and increments it each time a transmission is sent on the route set. lepim Specifies the local end-point incarnation master 414 to which this object belongs. max_r_a_tov The maximum value of the member routes' local_port_proxy.r_a_tov. remote_end_point_iid Incarnation identifier (IID) of the route set's remote end-point incarnation 410. remote_end_point_UID UID of the route set's remote end-point incarnation 410. route_index_keys Array of opaque keys used to verify dr_index values in received Transmissions. The route_proxy.local_route_index is used to index this array. On creation, the entries in this array are set to zero. When a route proxy 408 is added to the route set 600, the corresponding route_index_keys value is incremented and is copied into route_proxy. local_route_index_key. route_order_is_arbitrary True implies that the routes 300 within the route set 600 have not been ordered since the route membership last changed. On creation, this value is set to false. Subsequently, it is set to true each time a route is added to or remove from the route set. It is cleared when the routes within the route set are assigned an order. route_proxies Array of handles of the route proxies 408 that belong to this route set. The route_proxy.local_route_index is used to index this array. rtsmc The route set's management connection object. It may be stored either as a reference to a transport layer connection or as an object embedded in the route set object 417. send_initial_sfe_queue Queue of OBMs waiting for a route 300 to send an initial SFE on this route set 600. Used for route selection when no acceptable route exists. send_retry_sfe_queue Queue of OBMs waiting for a route 300 to send a retry SFE on this route set 600. Used to select a route when no acceptable route exists. send_start_mse_queue Queue of OBMs waiting for a route 300 to send a Start_MSE on this route set 600. Used to select a route when no acceptable route exists. significant_events Array of bits indicating which routes 300 of the route set 600 have experienced a significant event since the route was described. state State of the route set object 417. Route Management Connection Object

The route management connection 603 is a sequenced-message connection that provides a one-way sequenced message delivery service. Each message payload sent on an established route management connection 603 is prefixed with network, routing, and transport headers, as in the case of a ‘plain’ connection. All route management connection message payloads are received in the header buffer portion of a pool buffer.

The route management connection object 405 tracks the number of attempts to complete process login with retries_remaining, which, in an exemplary embodiment, is initially set to a value of 8. When retries_remaining reaches zero, the route management connection Connect Request operation fails. In response, route management deletes the corresponding pseudo remote end-point object 403 and remote port object 410.

Exemplary attributes associated with a route management connection (RMC) object 405 include a system buffer pool handle, a full route handle, retries_remaining, the RMC connection ID, and the state of the connection.

Route Monitoring

The routing layer 230 monitors transmissions on each half route 500(*) to determine the half route's quality. Routing layer 230 determines whether a half route 500 is delivering transmissions promptly, has lost recent transmissions, or is experiencing delay. This route quality information is used by a route selection algorithm (e.g., the algorithm of FIGS. 12A/B) to choose the best route 300 of a route set 600 for sending future transmissions.

More specifically, routing layer 230 monitors the quality of each inbound half route 500 using data supplied by the remote end-point's routing layer via the routing header that is included in each transmission. All half route quality information is combined into a single metric called the transmission rate limit (TRL) (See step 1610, FIG. 16). The TRL is computed by the receiving end-point's routing layer and is fed back to the transmitting end-point where it is used by the routing layer to place an upper bound on the rate of transmissions issued on the outbound half route. When not limiting, the rate of transmissions divided by TRL indicates the degree of outbound half route underutilization, a metric the routing layer uses to select a route for an initial transmission.

FIG. 17 is a diagram showing an exemplary routing layer feedback loop 1710/1715/1720/1725 that is executed to control the transmission rate for each half route 500 and to supply route quality information to be used by the transmitting routing layer for route selection.

In operation, transmissions 1710 are sent from transmitting end-point 102(1) via outbound half route 500 to receiving end-point 102(2). The routing layer at end-point 102(2) monitors the quality of each inbound half route 500 using data supplied by a transmitting end-point 102(1) via the routing header 1400 of each received transmission 1710. In an exemplary embodiment, all half route quality information is combined by the receiving routing layer at block 1715 (see step 1610, FIG. 16) to establish a transmission rate limit (TRL) 1404. Transmission rate limit determination is described in detail below.

After TRL 1404 is computed by the receiving end-point 102(2), it is fed back to the transmitting end-point in the dr_trlc field of the routing header 1400, via the next transmission 1720 that describes that half route 500, where it is used by the routing layer to place an upper bound on the rate of transmissions issued on the outbound half route 500.

In an exemplary embodiment, a routing layer program 1703 continually determines a current value for the transmission rate (TR) 1701 on each outbound half route by passing a sequence of inter-transmission times through a low-pass digital filter and inverting the result (see step 1523, FIG. 15A). Block 1725 functions as a limiter in which the transmitting end-point 102(1) compares the current value for TR 1701 with the value of TRL 1404 fed back from the remote end-point 102(2) to determine whether to send the next transmission on that outbound half route or another route within the route set, or to delay it. When the current TR 1701 is no greater than TRL 1404, the transmission is sent without imposition of a delay. In an exemplary embodiment, when the current TR 1701 exceeds TRL 1404, no transmissions will be sent on the outbound half route 500(1) because the route will not be acceptable and thus not selected (see FIG. 12). However, the next transmission may be sent on another route of the route set without delay if another route is acceptable. In the case where no route of the route set is acceptable, the OBM is queued as described in FIG. 13, at step 1315. As time passes, the intertransmission time will increase and thus the transmission rate 1701 of an unacceptable route will decrease until it reaches a value that is equal to or less than the present TRL 1404, at which time the route will become acceptable again. This mechanism effectively limits the transmission rate to be no greater than TRL.

To avoid continually computing a half route's transmission rate when its transmission rate exceeds its transmission rate limit, a timer is started whose expiration time is computed to be the time when TR will equal TRL. When the timer expires, TR is updated. Then, if the route is acceptable and OBMs are waiting, route selection is triggered, as described above with respect to FIGS. 12A/B and 13.

When not limiting, the transmission rate 1701 divided by the transmission rate limit 1404 indicates the degree of outbound half route underutilization. This metric is used by initial route selection at step 1245, in FIG. 12.

In the case where an end-point 102(*) detects a serious problem with an outbound half route, a mechanism is provided for communicating that event to a corresponding remote end-point 102(R). For example, if a transmission sent to the network layer 240 times out because the network layer detects so much congestion on the network that it cannot send the transmission within E_D_TOV (see FIG. 15A, step 1550), this mechanism allows the transmitting end-point to request that the remote end-point 102(R) set the inbound half route's TRL 1702 to one transmission per E_D_TOV.

Routing Sequence Numbers

The routing layer 230 counts transmissions sent on each outbound half route 500 from the time the full route 300 was established. These counts are called routing sequence numbers (RSNs) and they facilitate monitoring half routes 500. Each transmission on a route set 600 is uniquely identified by the route 300 on which it is sent, its direction or sending end-point 102, and its routing sequence number. The sending end-point 102 knows this information for each transmission.

In an alternative embodiment, each transmission includes its own RSN (i.e. the RSN of the outbound half route on which the transmission was sent), which allows both sending and receiving end-points 102 to share knowledge of each transmission's unique identity. In particular, upon receiving a transmission, the end-point 102 can readily determine whether any previous transmissions sent on the same route 300 are missing(i.e., known to have been sent but not yet received) by comparing the RSN just received to the highest RSN previously received.

The above concept may be extended across routes 300 in a route set 600. In an alternative embodiment, each transmission describes not just its own RSN, but the RSN for every outbound half route 500 in the route set 600. These RSNs identify the most recent transmission sent on each outbound half route 500. Together the RSNs identify all transmissions that have been sent in a particular direction on the route set 600 prior to the subject transmission. For each transmission that it receives, a receiving end-point 102 determines which prior transmissions it has received (on all routes of the route set) and which are missing.

The receiving end-point 102 cannot always distinguish a missing transmission as lost or delayed; both are transmissions that the receiving end-point 102 expects to receive but has not received. However, because Fibre Channel fabrics (and communication fabrics in general) are unlikely to deliver transmissions out of order when sent via a single route 300, the receiving end-point 102 can use the manner in which it learned of a missing transmission to distinguish those that were probably lost from those that were probably delayed.

When evidence of a missing transmission is obtained on the same route 300 as the missing transmission, then the transmission is probably lost. For example, suppose an end-point 102 had received all transmissions through RSN N on route A, and then received RSN N+5 on the same route. The end-point may then conclude that the four transmissions RSN N+1 through RSN N+4 were probably lost.

When evidence of a missing transmission is obtained on a different route 300 than the missing transmission, then the transmission is first considered probably delayed, and then after a predetermined period of time equal to R_A_TOV, it is considered lost. For example, suppose an end-point 102 has received all transmissions through RSN N on route A, and then received a transmission on route B specifying that the most recent RSN sent on route A was N+10. In this case, the receiving end-point may initially conclude that the ten transmissions N+1 through N+10 are probably delayed. If the end-point next received transmission N+5 on route A, it would then conclude that the four transmissions N+1 through N+4 were probably lost, the transmission N+5 was certainly delayed, and the five transmissions N+6 through N+10 are probably still be delayed. If no further transmissions are received on route A for R_A_TOV, the end-point may then conclude that transmissions N+6 through N+10 are certainly lost.

As each transmission is received, the receiving end-point 102 compares an RSN included in the transmission with that expected for the corresponding inbound half route 500. Thus, the end-point must store an expected RSN for each inbound half route. If it were not for lost transmissions, the expected RSN would simply be the count of transmissions received on the inbound half route 500. However, because transmissions may be lost, the expected RSN is estimated using the following procedure.

When a transmission is received that includes the RSN of the half route 500 on which the transmission was sent, the inbound half route's expected RSN is updated: If the received RSN is greater than or equal to the inbound half route's expected RSN, the inbound half route's expected RSN is set to one higher than the received RSN. Otherwise, the transmission must have been received out of order and the expected RSN is left unchanged.

Because out-of-order delivery on a single route 300 is unlikely, it is not necessary to include, in every transmission, the RSN of the half route 500 on which the transmission is sent. When a transmission is received that does not include the RSN of the half route 500 on which it was sent, the receiver simply increments the inbound half route's expected RSN. Furthermore, because changes in route quality tend to occur infrequently, it is not necessary to include, in every transmission, the RSN of every other outbound half route of the receive route's (RR) route set. In an exemplary embodiment, each transmission sent on outbound route RR includes the RSN of one of the outbound routes, the described route (DR), of the receive route's route set, and the described route is rotated among the routes of the route set.

Route Quality Metric Computation

In the present embodiment, in each transmission sent on a route set, the routing layer describes monitoring information for one route of that route set. The monitoring information is placed in the transmission's routing header. As previously indicated, for a given transmission, the route described in the routing header is called the described route (DR). In contrast, the route on which the transmission is sent and received is called the receive route (RR). The routing header includes information about the DR including the DR's identifier, the routing sequence number (RSN) of the last transmission sent on the outbound half DR, and the time since the last transmission was sent on the outbound half DR. The RR is determined by the route selection algorithm at the transmitter and passed to the network layer. At the receiver, the network layer supplies the RR as it passes the received transmission to the routing layer. The DR is normally rotated among the members of the route set at the transmitter, and determined by the receiver from the DR identifier included in the routing header.

Route monitoring processing is performed by the receiving end-point which examines the monitoring information contained in the routing header of each received transmission and determines a route quality metric for each route called the transmission rate limit or TRL. The basic algorithm for route monitoring uses routing sequence numbers to determine whether transmissions are being received promptly, are being lost, or are being delayed, using the basic approach described in the immediately previous section.

FIG. 18 is a diagram that provides an overview of an exemplary algorithm that uses routing sequence numbers to compute a route's TRL. In an exemplary embodiment, all RSN arithmetic is done modulo 2³². Specifically, the difference between two RSNs is computed by treating their unsigned difference as a signed value. Therefore, the difference between a half route's RSN and expected RSN is assumed to never exceed 2³¹−1.

As previously described, because Fibre Channel fabrics (and communication fabrics in general) are unlikely to deliver transmissions out of order when sent via a single route 300, a receiving end-point 102 can use the manner in which it learned of a missing transmission to distinguish those that were probably lost from those that were probably delayed. Note that a transmission is considered to be missing when it is determined to have been sent but has not yet been received. When evidence of a missing transmission is obtained on the same route 300 as the missing transmission, then the transmission is probably lost. When evidence of a missing transmission is obtained on a different route 300 than the missing transmission, then the transmission is first considered probably delayed, and then after a period of time equal to R_A_TOV, it is considered lost. Thus, the receiving end-point can compare RR to DR(at step 1815) to determine whether the transmission was probably lost or probably delayed.

As shown in FIG. 18, after a transmission is received, at step 1805, when RR=DR (at step 1815), and the received dr_rsn exceeds the expected RSN for DR (at step 1820), then there are probably X=received dr_rsn_expected RSN transmissions lost on this route, as concluded at step 1830. The intermediate variable X is computed at step 1810. Normally, the received dr rsn equals the expected RSN, which is the case when X=0. When the received dr_rsn is less than the expected RSN, then a transmission was delivered out of order on the route, and thus a transmission previously determined to have been lost was instead delayed, as concluded at step 1835. These conclusions are then used to compute, for the inbound half route, the TRL as well as the intermediate state variables TLL, DSS, TDL, and the expected RSN, at step 1850.

When RR differs from DR, and the received dr_rsn equals or exceeds the expected RSN for DR (at step 1825), then there are probably X+1 transmissions delayed on this route (as concluded at steps 1840 and 1845). Normally, the received dr_rsn is one less than the expected RSN because the transmission being described should have already been received on the DR and that would have increased the expected RSN for DR to one greater than the dr_rsn. When dr_rsn is less than the expected RSN by more than one, there is a weak implication that the RR is slower than the DR, but because that implication is weak it is ignored. These conclusions are also used to compute, for the inbound half route, the TRL as well as the intermediate state variables TLL, DSS, TDL, and the expected RSN at step 1850. Processing of the received transmission then continues at step 1855.

Fundamental to the algorithm described in FIG. 18 is the ability to compute the expected RSN for each inbound half route of a route set. FIG. 19 is a flowchart illustrating an exemplary method for determining expected RSNs. As described earlier, when a transmission is received that includes the RSN of the half route 500 on which the transmission was sent, the inbound half route's expected RSN is updated: If the received RSN is greater than or equal to the inbound half route's expected RSN, the inbound half route's expected RSN is set to one higher than the received RSN. Otherwise, the transmission must have been received out of order and the expected RSN is left unchanged. This is accomplished by comparing RR to DR in step 1915. The normal case when RR=DR is that the received RSN equals the expected RSN (at step 1920), in which case X computed in step 1910 will be zero and the expected RSN will be increased by 1 at steps 1935 & 1940. When RR=DR and the received RSN exceeds the expected RSN then X transmissions were probably lost and so the next transmission expected on this route will have an RSN equal X+1 (done at steps 1935 and 1940). When RR=DR and the received RSN is less than the expected RSN, then the transmission was received out of order, but the expected RSN should not change (steps 1930 and 1940). When a transmission is received where DR differs from RR then we assume that this is the transmission expected on RR and thus increase the RSN for the RR by 1, at steps 1925 and 1940. Processing of the received transmission then continues at step 1945.

Calculation of Transmission and Reception Rate

The routing layer monitors each outbound half route's transmission rate (TR) and each inbound half route's reception rate (CR). This transmission rate is compared to the outbound half route's transmission rate limit, as described previously, to determine whether the route is acceptable for transmission, and the reception rate is used in the computation of TRL. Both TR and CR are computed by passing a sequence of inter-transmission times through a low-pass digital filter. Specifically, the outbound half route tracks the time when the last transmission was issued, last_send_time, and estimates TR, as follows. transmission_iat_est_(i)=(1−TR_FILTER)*transmission_iat_est_(i-1) +TR_FILTER*(current_time−last_send_time) TR _(i) =IRR_SCALE/transmission_iat_est_(i)

Note that TR, transmission_iat_est, and last_send_time are updated during the last step of route selection rather than at the time the transmission is sent, because there can be a significant delay between route selection and transmission. Thus, updating TR at the end of route selection assures that each decision will be factored into the next route selection.

Similarly, the inbound half route tracks the time when the last transmission was received, last_receive_time, and estimates CR, as follows: reception_iat_est_(i)=(1−SCR _(—) TR_FILTER)*reception_iat_est_(i-1) +SCR _(—) TR_FILTER*(current_time−last_receive-time) CR _(i) =SCR _(—) IRR_SCALE/reception_iat_est_(i)

Note that while the reception_at_est and last_receive_time are updated for each received transmission, CR is only updated when TLL is increased to avoid the expense of the divide operation.

Before concluding that TR exceeds TRL, the estimated TR must be calculated at current_time, as determined by the transmitting end-point's clock. A timer is used to limit these calculations and to trigger route selection for waiting OBMs. When a route's TR exceeds its TRL, a timer is started whose expiration time is computed to be when TR will equal TRL. When the timer expires, the transmission rate is updated. Then, if the route is acceptable and OBMs are waiting, route selection is triggered.

Transmission Delay Likelihood and Transmission Loss Likelihood

The description of FIG. 18 indicates the basic strategy for using the RSNs in received routing headers to make conclusions concerning whether the described route is losing or delaying transmissions. Intuitively, a route's quality metric should be decreased when there is recent evidence of transmission loss or delay on that route, and it should be increased when there is recent evidence of prompt and reliable transmission delivery on that route. Because the present system uses TRL to express half route quality and because TRL is incorporated in a feedback loop, it is not necessary to establish a linear relationship between TRL and recent transmission loss or delay. Instead, the present invention uses a heuristic algorithm to relate transmission loss and delay to TRL. This algorithm computes a transmission loss likelihood (TLL) and a transmission delay likelihood (TDL) for each half route. These are functions of time that increase when there is evidence of recent transmission loss or delay. As increases in these functions cross thresholds, the inbound half route TRL is decreased.

The loss of a single transmission is not significant enough to alter routing decisions. However, the recent loss of multiple transmissions is. Intuitively, the more transmissions that appear to have been lost and the more recently they appear to have been lost, the more certainly the quality metric of the half route should be lowered.

In practice, the receiver uses routing header information to compute a transmission loss likelihood for the inbound half route, and when a received transmission causes transmission loss likelihood to increase beyond a threshold it reduces the inbound half route's TRL. Transmission loss likelihood (TLL) is computed as the output of a low-pass digital filter in which a TLL input sequence value occurs each time a transmission is received in which RR equals DR. The TLL input sequence value is the maximum of zero and the difference, received RSN less expected RSN.

As noted previously, the routing header data allows the receiver to conclude when the inbound half DR is probably slower than the inbound half RR. Intuitively, the more transmissions that appear to have been delayed, the larger the delay, and the more recently they appear to have been delayed, the more certainly the quality metric of the inbound half route should be lowered. This notion is captured in a heuristic metric called transmission delay likelihood.

In practice, the receiver also uses routing header information to compute a transmission delay likelihood for the inbound half route, and when a received transmission causes transmission delay likelihood to increase beyond one of several thresholds it reduces the inbound half route's TRL. Transmission delay likelihood (TDL) is computed as the output of a low-pass digital filter in which a TDL input sequence value occurs each time a transmission is received in which RR differs from DR. The TDL input sequence value is another function of time called delay skew sum (DSS) which approximates the product of the number of delayed transmission on the inbound half route and their mean delay time.

TDL reacts to lost and delayed transmissions as follows. Mild congestion on a route will cause delays in some transmissions, which will cause the route's TDL to rise slightly. Heavy congestion on a route will cause transmissions to be discarded, which will cause the route's TDL to rise significantly and remain high until that route describes itself. Lost transmissions due to CRC errors cannot be distinguished from those lost due to congestion, and thus have the same effect on TDL.

When a route fails (i.e., all transmissions on the route are lost), the transmission loss likelihood (TLL) will remain constant because TLL is updated only when a transmission is received that describes the receive route (RR). However, as long as another route within the route set continues to function, transmissions will be sent that describe the failed route and consequently its TDL will continue to rise. This fact will enter the TRL computations and discourage the use of the failed route.

Because TLL detects lost transmissions only after a transmission is received that describes RR, TDL usually detects lost transmissions before TLL does. However, TLL is not necessarily redundant, because a rise in TDL may be due to a delayed transmission, and thus increases in TLL indicate lost transmissions with greater certainty and thus warrant more deliberate reductions in transmission rate limit. Furthermore, in the case where a there is only one route, TDL is never computed and thus TLL is the only mechanism for detecting lost transmissions.

Calculation of Transmission Loss Likelihood

FIG. 20 is a flowchart illustrating an exemplary method of computing transmission loss likelihood and how TLL is used to modify the transmission rate limit. Note that TLL is only updated upon receipt of a transmission in which DR=RR. As shown in FIG. 20, at step 2005, a temporary variable ‘X’ is set equal to the value of the difference between the RSN of the route described in the routing header of the received transmission (dr_rsn) and the expected RSN of the described route (DR). The value of TLL for the inbound half route is updated for the DR with input X in accordance with the steps shown in block 2010, as follows.

At step 2015, transmission loss likelihood (TLL) is computed as the output of a low-pass digital filter in which a TLL input sequence value occurs each time a transmission is received in which the RR equals the DR. The digital filter is expressed for a TLL-input value of X, as follows: TLL _(i)=(1−TLL_FILTER)*TLL _(i-1) +TLL_FILTER*TLL_input_(i) TLL_FILTER is a first-order digital filter constant used to smooth TLL. An exemplary value for TLL_FILTER is 1/32.

At step 2020, the calculated value for TLL is constrained to the interval from zero to TLL_MAX. An exemplary value for TLL_MAX is 2²⁶ −1. At step 2025, if the value of X is not greater than zero, then the transmission rate limit remains unchanged, and a return is taken at step 2050. If X>0, then at step 2030, the current value of TLL is compared to TLL_THRESHOLD. TLL_THRESHOLD is a value that is compared to TLL to detect when TLL has become large enough that the corresponding transmission rate limit should be reduced. An exemplary value for TLL_THRESHOLD is 66. When TLL is increased and its value is greater than TLL_THRESHOLD, the corresponding transmission rate limit is reduced by (multiplied by) TRL_DECREMENT_FRACTION, at step 2035. In an exemplary embodiment, TRL_DECREMENT_FRACTION has a value of 0.5. However, if the inbound half route's reception rate is already lower than its TRL then TRL should instead be set to the reception rate multiplied by TRL_DECREMENT_FRACTION.

At step 2040, the current value for TRL is constrained to the range [max (reception_rate,TRL_MIN), TRL_MAX], where TRL_MIN equals 0.625 transmissions per second, and TRL_MAX is 1,048,576 transmissions per second, in an exemplary embodiment. At step 2045, the corresponding route's significant event flag is set, and a return is taken at step 2050.

Transmission Delay and Delay Skew Sum

As noted previously, routing header data allows the receiver to conclude when the inbound half DR is probably slower than the inbound half RR. Intuitively, the more transmissions that appear to have been delayed, the larger the delay, and the more recently they appear to have been delayed, the more certainly the quality metric of the inbound half route should be lowered. This notion is captured in the transmission delay likelihood (TDL) metric.

FIG. 21 is a diagram illustrating how transmission delay time can be determined when a single transmission is delayed. In FIG. 21, transmission 2110 is sent on route RR and describes route DR. In this case, transmission 2105, which was sent on route DR is delayed so that it is received after transmission 2110. As shown in FIG. 21, when a single transmission 2105 is delayed on a half route, its delay time is defined as the inter-transmit time 2101 plus the inter-receive time 2102. Inter-transmit time 2101 is the time between the transmission's transmit time and the transmit time of the transmission 2110 that described it. Inter-receive time 2102 is the time between when the transmission that described the delayed transmission was received and when the delayed transmission was actually received. Thus, transmission delay time indicates how much slower the DR is than the RR.

Several factors complicate this simple definition of transmission delay time:

-   -   When multiple transmissions are delayed on a half route, the         definition of delay time is less clear because the transmitter         cannot afford to store and send every transmission's         inter-transmit-time and the receiver cannot afford to store         every transmission's receive time. Therefore, the receiver must         estimate inter-transmit-time for all but the most recent         transmission on the DR.     -   Multiple transmissions may describe the same half route and thus         describe the same delayed transmissions. These transmissions may         have been sent on one or multiple routes and may be received in         any order. To avoid double counting, the routing layer tracks         the highest described RSN for each route.     -   A transmission delay time can be computed for each pair of         routes, but that computation can be expensive. To avoid the         expensive computation, a heuristic metric called delay skew sum         is calculated for each route.

The heuristic metric, delay skew sum, is defined as the product of the number of transmissions currently believed delayed on a half route and their approximate delay times. Another metric, mdter_time, is an approximation for the mean delayed transmission expected receive time, so the delay skew sum is approximated by the product of the number of transmissions believed to be delayed on the half route and the difference between the current time and mdter_time.

When RR=DR and the received RSN>highest described RSN, then it is highly likely that transmissions previously thought to have been delayed are now lost. This fact will be reflected in TLL, so mdter_time is set to the current time to remove the influence of those transmissions on TDL.

When RR differs from DR and the received dr_rsn exceeds the highest described RSN for DR, then the transmission described is one that has not been described before. FIG. 22 is a flowchart showing an exemplary method for calculating mdter_time in this case. As shown in FIG. 22, at step 2205, a temporary variable ‘X’ is set equal to the received dr_rsn minus the expected RSN for the DR, and temporary variable ‘Y’ is set equal to the highest described RSN for the DR minus the expected RSN for the DR. X+1 is the total number of transmissions believed to have been delayed or lost on the half route and Y+1 is the number of transmissions believed to have been delayed or lost on the half route prior to the receipt of this transmission.

The value for mdter_time is then updated with inputs X and Y, as shown in block 2210. Note that the steps shown in block 2310 are executed only when X is non-negative.

When Y+1≦0, which is tested at step 2215, one can conclude that immediately prior to receipt of this transmission, there was no evidence of delayed transmissions, so mdter_time can be calculated simply using the method described in FIG. 21. That is, at step 2225, the DR's mdter_time is updated by subtracting the received inter-transmit time for the DR(dr_itt) from the current time. Subsequently, the delay time will grow as the current time minus mdter_time.

When Y+1<0, again tested at step 2215, then the current transmission is providing evidence of additional delayed or lost transmissions, and in this case the DR's mdter_time is updated at step 2220 as follows: current_time−(mdter_time−current_time)*(Y+1)/(X+1) Calculation of Transmission Delay Likelihood

FIG. 23 is a flowchart showing an exemplary method for calculating transmission delay likelihood and how it is used to modify the transmission rate limit. As shown in FIG. 23, transmission delay likelihood is computed for the described route (DR) by converting the delay skew sum (DSS) to a sequence and then filtering the sequence with a low-pass digital filter.

At step 2305, a temporary variable ‘Y’ is set equal to the value of the difference between the highest described RSN for the described route and the expected RSN of the described route. As before, Y+1 is the number of transmissions believed to be delayed on the inbound half route. The value of TLL for the described route with input Y+1 is then updated in accordance with the steps shown in block 2310, as follows.

At step 2315, if Y+1 is less than or equal to zero, then the received transmission indicates that there are no longer any delayed transmissions on the described inbound half route and thus the value for delay skew sum sequence variable DSS_(i) is set to zero (at step 2320), otherwise, DSS_(i) is set equal to (current_time−mdter_time)*(Y+1), at step 2325. Thus, DSS is the product of the number of transmissions believed to be delayed and the approximate mean time that they are believed to be delayed. In either case, the value for transmission delay limit element TDL_(i) is then calculated at step 2330 by the following low-pass digital filter: TDL _(i)=(1−TDL_FILTER)*TDL _(i-1) +TDL_FILTER*DSS_(i) At step 2335, TDL is limited to a range between [0, TDL_MAX], where TDL_MAX has an exemplary value of 2²⁶−1. If Y is greater than zero, and └TDL_(i)/TDL_THRESHOLD┘>└TDL_(i-1)/TDL_TRESHOLD┘, where TDL_TRESHOLD has an exemplary value of 1024, at steps 2340 and 2345, respectively. TRL is then adjusted as described below, starting with step 2350; otherwise, the current value for TRL is left unchanged, and a return is taken, at step 2365.

At step 2350, the value for TRL is reduced by a value of TRL_DECREMENT_FRACTION, which has an exemplary value of 0.5. At step 2355, TRL is limited to a range of [max (reception_rate,TRL_MIN), TRL_MAX]. At step 2360, the route's significant event flag is set, and a return is taken at step 2365.

FIG. 24 is a graphical example of a delay skew sum calculation showing two delayed transmissions. In the FIG. 24 graph, time t increases in the downward direction. In the present example, events occur in the following order:

-   -   1. A transmission 2405 is sent on route DR. Assume that the RSN         of this transmission be 101.     -   2. A transmission 2410 is sent on route DR. Assume that the RSN         of this transmission be 102.     -   3. A transmission 2415 is sent on route RR that describes route         DR in its routing header.     -   4. Transmission 2415 is received on route RR at time s.     -   5. Transmission 2405 is received on route DR. It was received         after time s because it was delayed by route DR.     -   6. Transmission 2410 is received on route DR. It was received         after time s because it was delayed by route DR.

In the present example, assume that prior to sending transmission 2405, the expected RSN for DR is 101 and the highest described RSN for DR is 100. When transmission 2415 is received at time s, it includes in the routing header dr_rsn=102 and dr_itt=a. Thus, mdter_time=s−a. That is, mdter_time is the time when transmission 2410 would have arrived had it not been delayed. The highest described RSN for DR changes to dr_rsn=102, and the apparent number of delayed transmissions on DR is 2. That is, new highest described RSN for DR—expected RSN for DR+1=102−101+1=2. This yields a DSS of 2*a at step 2425.

Between time s and the receipt of transmission 2405, DSS increases linearly with time as 2*(a+t−s), which is the equation describing a line that intersects the t axis at s−a and yields a DSS of 2*a at time s. Upon the receipt of transmission 2405, the expected RSN for DR increases to 102 and thus the new number of delayed transmissions on the DR changes to highest described RSN for DR—expected RSN for DR+1=102−102+1=1.

Between the receipt of transmission 2405 and the receipt of transmission 2410, there is one delayed transmission and thus the DSS increases linearly with time as 1*(a+t−s), which is the equation describing a line that intersects the t axis at s-a and which would yield a DSS of 1*a at time s. Upon the receipt of transmission 2410, the expected RSN for DR increases to 103, and thus the number of delayed transmissions on DR and DSS change to zero.

Detailed Algorithm

FIG. 25 is a flowchart showing an expanded exemplary algorithm, encompassing the previously described components, for determining transmission-related factors in the present system. As shown in FIG. 25, after a transmission is received at step 2505, two temporary variables are initialized, at step 2510. First, a temporary variable ‘X’ is set equal to the value of the difference between the received RSN for the described route (dr_rsn) and the expected RSN of the described route. Next, a temporary variable ‘Y’ is set to the value of the difference between the highest described RSN for the DR minus the expected RSN for the DR. The variables X and Y are the same temporary variables previously described in steps 1810, 1910, 2005, 2205, and 2305.

At step 2515, if the receive route and the described route are the same route, and if the received dr_rsn is greater than the highest described RSN for the DR (at step 2520), mdter_time for the DR is set to the current time (for the delay skew sum calculation), at step 2521. At step 2522, the highest described RSN for the DR is set equal to the received dr_rsn. If the RR and the DR are the same route, but the received dr_rsn is not greater than the highest described RSN for the DR, then processing continues at step 2523.

At step 2523, if X<0, then the described route TLL is updated as described in FIG. 20 with input 0 at step 2525, and the expected RSN increment is set to zero, at step 2528. If X≧0 (at step 2523), then the described route TLL is updated as described in FIG. 20 with input X at step 2524, the described route TDL is updated as described in FIG. 23 with input Y at step 2526, and the expected RSN increment is set to a value of X+1, at step 2527.

At step 2540, the expected RSN for the RR is increased by the expected RSN increment; and at step 2545, the TRL for the RR is increased for the RR, and the reception rate estimate for the RR is updated. Increases in TLL and TDL are used to decrease TRL, and successfully receiving a transmission on RR is used to increase TRL. Here, the value for TRL is increased by a factor of TRL_INCREMENT_FRACTION (approximately 19% in an exemplary embodiment in which it represents an increment of exactly 2 in the compressed TRL table), and the TRL range is limited to the interval [TRL_MIN, TRL_MAX]. Processing of the received transmission then continues at step 2550.

If, at step 2515, the receive route and the described route are not the same route, then if the received dr_rsn is greater than the highest described RSN for the DR, and X≧0, at steps 2530 and 2531, respectively, then mdter_time for the DR with X and Y is updated (for the DSS calculation) as described in FIG. 22, at step 2532. If X<0 (at step 2531), the mdter_time is not updated. At step 2533, the highest described RSN for DR is set equal to the received dr_rsn. If the received dr_rsn is not greater than the highest described RSN for the DR (at step 2530), then steps 2532/2533 are skipped.

At step 2534, Y is set equal to the difference between the new highest described RSN and the expected RSN for the DR, and the described route TDL is updated with input Y+1, at step 2535 as described in FIG. 23. The expected RSN increment is set to a value of 1, at step 2536, and processing continues at step 2540, as described above.

Certain changes may be made in the above methods and systems without departing from the scope of the present system. It is to be noted that all matter contained in the above description or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense. For example, the network shown in FIG. 1 may be constructed to include components other than those shown therein, and the components may be arranged in other configurations. The elements and steps shown in FIGS. 3-25 may also be modified in accordance with the methods described herein, without departing from the spirit of the system thus described. In addition, fabric 105(*) may be any type of switched network, including the Internet. 

1. A method for determining route quality in a network in which a plurality of routes exist between a transmitting end-point and a receiving end-point comprising the steps of: associating a routing sequence number with each transmission sent on a route in the network, such that consecutive transmissions sent on one of the routes have consecutive routing sequence numbers; including the routing sequence number, for a first said route, in transmissions sent on a second said route; using the routing sequence number of the first said route, received on the second said route, as an indicator of the route quality of the first said route; and comparing the indicator to an expected routing sequence number for the first said route to determine a likelihood of whether the transmissions sent via the first said route are either lost or delayed; wherein the expected routing sequence number is normally incremented each time a transmission is received on the second said route.
 2. A method for determining relative route quality in a network in which a plurality of routes exist between a transmitting end-point and a receiving end-point comprising the steps of: associating a routing sequence number with each transmission sent on one of the routes such that consecutive said transmissions sent on one of the routes have consecutive routing sequence numbers; including the routing sequence number and route identifier of at least one of the plurality of routes in each of the transmissions sent on each said route, wherein the route accordingly described in each sent transmission is the described route; computing the expected routing sequence number for each of the routes received at the receiving end-point; and using the difference between the routing sequence number included in a received transmission and the expected routing sequence number for the corresponding described route as an indicator of the described route's quality.
 3. The method of claim 2, wherein the route on which a given said transmission is sent and received is the receive route, and wherein the receive route's expected routing sequence number is computed by performing steps comprising: increasing the expected routing sequence number by 1, if a received transmission describes the routing sequence number of the receive route, and if the routing sequence number described in the received transmission equals the current expected routing sequence number of the described route; increasing the expected routing sequence number by a value of N+1, if the received transmission describes the routing sequence number of the receive route and the described routing sequence number exceeds the current expected routing sequence number of the described route by a value of N; and increasing the expected routing sequence number by 1, if the received transmission does not describe the routing sequence number of the receive route.
 4. The method of claim 3, wherein a routing sequence number described in the received transmission equals or exceeds the expected routing sequence number for the corresponding described route constitutes an indication of at least one missing transmission on the described route, unless the described route is the receive route and the described routing sequence number equals the expected routing sequence number.
 5. The method of claim 3, wherein the likelihood of whether a missing transmission is lost or delayed is determined as a function of the route on which an indication of the missing transmission is received; wherein: when the indication of the missing transmission is obtained on the same route as the missing transmission, then the missing transmission is determined to be probably lost; and when the indication of the missing transmission is obtained on a different route than the route on which the missing transmission should have been received, then the missing transmission is determined to be probably delayed initially, and after a predetermined period of time, the missing transmission is determined to be lost.
 6. The method of claim 3, wherein, when one of the transmissions is received that includes a routing sequence number for the receive route, then transmissions are determined to be missing, by calculating the difference between the value of the routing sequence number described in the transmission and the value of the expected routing sequence number for the described route, wherein: if the routing sequence number described in the transmission exceeds the expected routing sequence number for the described route by a value of N, then N transmissions are determined to be missing on the described route and probably lost; and if the routing sequence number described in the transmission is less than the expected routing sequence number for the described route, then a transmission previously determined to have been lost on the described route was instead delayed.
 7. The method of claim 3, wherein the route on which the transmission is sent and received is the receive route, and wherein, when one of the transmissions is received that includes a described routing sequence number for one of the routes other than the receive route, then one or more of the transmissions is determined to be missing by performing steps comprising: determining that the transmission being described is missing on the described route and probably delayed, if the described routing sequence number equals the expected routing sequence number for the described route; and determining that N+1 said transmissions are missing and probably delayed, if the described routing sequence number exceeds the expected routing sequence number for the described route by N.
 8. A method for determining relative route quality in a network in which a plurality of routes exist between a transmitting end-point and a receiving end-point comprising the steps of: associating a routing sequence number with each transmission sent on one of the routes such that consecutive said transmissions sent on one of the routes have consecutive routing sequence numbers; including the routing sequence and number route identifier of a single one of the plurality of routes in each of the transmissions sent on each said route, wherein the route accordingly described in each sent transmission is the described route; rotating the described route among the plurality of routes in the transmissions sent on said routes selected from the plurality of routes; computing an expected routing sequence number for each of the plurality of routes at the receiving end-point; using the difference between the routing sequence number included in a received transmission and the expected routing sequence number for the corresponding described route as an indicator of the described route's quality; and using the indicator to determine a likelihood of whether the transmissions sent via the described route are either lost or delayed.
 9. The method of claim 8, wherein the route on which a given said transmission is sent and received is the receive route, and wherein the receive route's expected routing sequence number is computed by performing steps comprising: increasing the expected routing sequence number by 1, if a received transmission describes the routing sequence number of the receive route and described routing sequence equals the current expected routing sequence number of the described route; increasing the expected routing sequence number by a value of N+1, if the received transmission describes the routing sequence number of the receive route and the described routing sequence number exceeds the current expected routing sequence number of the described route by a value of N; and increasing the expected routing sequence number by 1, if the received transmission does not describe the routing sequence number of the receive route.
 10. The method of claim 9, wherein, when a routing sequence number described in the received transmission equals or exceeds the expected routing sequence number for the corresponding described route, a determination is made that at least one transmission is missing on the described route, unless the described route is the receive route and the described routing sequence number equals the expected routing sequence number.
 11. The method of claim 9, wherein the likelihood of whether a missing transmission is lost or delayed is determined as a function of the route on which an indication of the missing transmission is received, wherein: when the indication of the missing transmission is obtained on the same route as the missing transmission, then the missing transmission is determined to be probably lost; and when the indication of the missing transmission is obtained on a different route than the route on which the missing transmission should have been received, then the missing transmission is initially determined to be probably delayed.
 12. The method of claim 11, wherein, when one of the transmissions is received that describes a routing sequence number for the receive route, then certain ones of the transmissions are determined to be missing by calculating the difference between the value of the routing sequence number described in the transmission and the value of the expected routing sequence number for the described route, wherein: if the routing sequence number described in the transmission exceeds the expected routing sequence number for the described route by a value of N, then N transmissions are determined to be missing, and probably lost, on the described route; and if the routing sequence number described in the transmission is less than the expected routing sequence number for the described route, then a transmission that was previously determined to have been lost on the described route is presently determined to have been delayed.
 13. The method of claim 11, wherein, when one of the transmissions is received that includes a routing sequence number for a route other than the receive route, then one or more of the transmissions is determined to be missing by performing steps comprising: determining that the transmission being described is missing on the described route and probably delayed; and determining that N+1 said transmissions are missing and probably delayed, if the described routing sequence number exceeds the expected routing sequence number for the described route by N.
 14. The method of claim 9, wherein a non-negative metric constituting transmission loss likelihood is computed for each specific route, a high value of which indicates multiple recently lost transmissions on the specific route, and a low value of which indicates few recently lost transmissions on the specific route, wherein the metric is updated each time a transmission is received that describes a routing sequence number for the receive route, by performing the step of: if the described routing sequence number exceeds the expected routing sequence number for the described route by a value of N, then N transmissions are determined to be probably lost on the described route and the described route's transmission loss likelihood is updated with a low-pass digital filter with input N, otherwise the described route's transmission loss likelihood is updated with a low-pass filter with input of zero.
 15. The method of claim 14, wherein the metric for the specific route is decreased each time the specific route's transmission loss likelihood is updated and the new value is greater than a fixed threshold, and the metric for the specific route is increased each time a transmission is successfully received on the specific route.
 16. The method of claim 9, wherein a non-negative transmission delay likelihood metric is computed for each specific route, a high value of the metric indicating multiple recently delayed transmissions or transmissions recently delayed for long times on the specific route, and a low value of the metric indicating few recently delayed transmissions on the specific route, wherein, when a transmission is received that describes the routing sequence number for a different route other than the receive route and the highest routing sequence number described for the different route equals or exceeds the described route's expected routing sequence number by Y, then the transmission delay likelihood for the different route is updated with a low-pass digital filter with input (Y+1)*(current time−mean delayed transmission expected receive time).
 17. The method of claim 9, wherein a non-negative transmission delay likelihood metric is computed for each specific route, a high value of the metric indicating multiple recently delayed transmissions or transmissions recently delayed for long times on the specific route, and a low value of the metric indicating few recently delayed transmissions on the specific route, wherein, when a transmission is received that describes the routing sequence number for the receive route and the described routing sequence number equals or exceeds the receive route's expected routing sequence number, and the receive route's highest described routing sequence number is less than the expected routing sequence number, then the route's transmission delay likelihood is updated with a low-pass digital filter with input zero.
 18. The method of claim 9, wherein a non-negative transmission delay likelihood metric is computed for each specific route, a high value of the metric indicating multiple recently delayed transmissions or transmissions recently delayed for long times on the specific route, and a low value of the metric indicating few recently delayed transmissions on the specific route, wherein, when a transmission is received that describes a routing sequence number for the receive route, and the described routing sequence number equals or exceeds the receive route's expected routing sequence number, and the highest described routing sequence number described prior to the received transmission equals or exceeds the expected routing sequence number by Y, then the receive route's transmission delay likelihood is updated with a low-pass digital filter with input (Y)*(current time minus mean delayed transmission expected receive time); wherein mean delayed transmission expected receive time is approximated as current time minus a constant.
 19. The method of claim 9, wherein a mean delayed transmission expected receive time metric is computed for each said route, such that the metric approximates the mean of the expected receive times for delayed transmissions on a given said route, wherein the metric is updated when a transmission is received that describes a routing sequence number for a route other than the receive route and the described route's highest routing sequence number described prior to the receipt of the received transmission is one less than the expected routing sequence number therefor, and the described routing sequence number equals or exceeds the described route's expected routing sequence number by X, then the route's mean delayed transmission expected receive time is calculated as the current time minus the inter-transmission time for the route described in the received transmission.
 20. The method of claim 9, wherein a mean delayed transmission expected receive time metric is computed for each said route, such that the metric approximates the mean of the expected receive times for delayed transmissions on a given said route, wherein the metric is updated when a transmission is received that describes a routing sequence number for a route other than the receive route and the described route's highest routing sequence number described prior to the receipt of the received transmission is one less than the expected routing sequence number therefor, and the described routing sequence number equals or exceeds the described route's expected routing sequence number by X, then when a transmission is received that describes a routing sequence number for a different route other than the receive route and the described routing sequence number exceeds the described route's highest routing sequence number described prior to the receipt of the received transmission and the described routing sequence number equals or exceeds the described route's expected routing sequence number by X, then the different route's mean delayed transmission expected receive time is calculated as the current time−(mean delayed transmission expected receive time−current time)*(Y+1)/(X+1).
 21. The method of claim 9, wherein a mean delayed transmission expected receive time metric is computed for each said route, such that the metric approximates the mean of the expected receive times for delayed transmissions on a given said route, wherein the metric is updated when a transmission is received that describes a routing sequence number for a route other than the receive route and the described route's highest routing sequence number described prior to the receipt of the received transmission is one less than the expected routing sequence number therefor, and the described routing sequence number equals or exceeds the described route's expected routing sequence number by X, then when a transmission is received that describes a routing sequence number for the receive route and the described routing sequence number exceeds the highest routing sequence number described prior to the receipt of this transmission, then all transmissions thought to have been delayed previously are probably lost, and the mean delayed transmission expected receive time is set to the current time.
 22. The method of claim 19, wherein a route's quality metric is decreased each time the route's transmission delay likelihood is increased such that its value crosses a multiple of a fixed threshold and a route's quality metric is increased each time a transmission is successfully received on the route.
 23. A system for determining route quality in a network in which a plurality of routes exist between a transmitting end-point and a receiving end-point comprising: a routing sequence number, associated with each transmission sent on a route in the network, such that consecutive transmissions sent on one of the routes have consecutive routing sequence numbers; wherein: the routing sequence number, for a first said route, is included in transmissions sent on a second said route; the routing sequence number of the first said route, received on the second said route, is used as an indicator of the route quality of the first said route; and the indicator is compared to an expected routing sequence number for the first said route to determine a likelihood of whether the transmissions sent via the first said route are either lost or delayed; wherein the expected routing sequence number is normally incremented each time a transmission is received on the second said route.
 24. A software product comprising instructions, stored on computer-readable media, wherein the instructions, when executed by a computer, perform steps for determining route quality in a network in which a plurality of routes exist between a transmitting end-point and a receiving end-point comprising: associating a routing sequence number with each transmission sent on a route in the network, such that consecutive transmissions sent on one of the routes have consecutive routing sequence numbers; including the routing sequence number, for a first said route, in transmissions sent on a second said route; using the routing sequence number of the first said route, received on the second said route, as an indicator of the route quality of the first said route; and comparing the indicator to an expected routing sequence number for the first said route to determine a likelihood of whether the transmissions sent via the first said route are either lost or delayed; wherein the expected routing sequence number is normally incremented each time a transmission is received on the second said route. 